A question that comes up a fair bit on literature forums is: "How many books has Author X sold?". Compared to TV, where audience figures are easily available, and movies, where box office figures are even more easily retrieved, book sales figures are virtually impossible to calculate for an interested member of the public. The advent of the BookScan system in the USA has made this slightly easier, but the system is relatively new (introduced in 2001), it doesn't pick up every sale (according to Nielsen it tracks about 70% of sales) and only works in the USA (whilst more than half of the sales of titles take place outside that market). The New York Times and the UK Times offer their own figures, but refuse to disclose how those figures are reached. Publishers generally don't publish figures at all unless the book gets picked up for a movie option, or if the sales reach phenomenal levels. And of course often when figures are given they are for 'books in print' (i.e., the total number of copies of a book that exist, including those sitting unsold on bookshelves) rather than for books actually put through the till. Add to this the recent upswing in pirating books online, and the number of books where illegal or untracked editions have been printed in nations with a relaxed attitude to copyright, and you can see the difficulties faced in assembling any kind of all-time bestseller list.
For that reason, the following list should be taken with a grain of salt the size of Lake Michigan (the source for most of the figures is Wikipedia, unless otherwise noted):
1) J.K Rowling (350 million)
The Harry Potter series has been a phenomenon the likes of which publishing has never seen. In less than a decade, Rowling went from an impoverished single mother writing in an Edinburgh cafe to one of the richest women in the world, overtaking dozens of writers who had been working for decades in the process.
2) Stephen King (350 million)
In The Encyclopedia of Fantasy (1996), it was stated that Stephen King's total worldwide sales in all languages are probably incalculable, and the figure given above is on the conservative side of things. I've seen some figures suggesting he has sold twice this amount, but the 350m figure seems to crop up most often. Some may argue that Horror isn't necessarily part of the SF&F genre either and King shouldn't be counted, but most of his horror features supernatural forces, which firmly places it as a subset of Fantasy. Also, no-one would really argue that Eyes of the Dragon and the Dark Tower series aren't fantasy, and both of these works are set in the same multiverse as most (or, as some fans argue, all) of his other books, which puts him firmly in the Fantasy genre.
3) JRR Tolkien (c. 300 million)
Tolkien's sales really are incalculable, given how widely his books have been copied, published without permission and distributed worldwide in the last fifty years. However, it is pretty clear that by itself The Lord of the Rings is the biggest-selling single genre novel of all time, and possibly the biggest-selling single novel full stop of all time. 50 million copies of the novel have been sold this century alone. When you factor in the massive sales of The Hobbit, and the smaller but still significant sales of The Silmarillion, Unfinished Tales and The Children of Hurin, plus his non-Middle-earth work, Tolkien is clearly a major force in SF&F publishing, arguably all the more notable as his output was small compared to some others on this list.
4) CS Lewis (120 million)
It is perhaps fitting that Tolkien's one-time best friend and sometimes collaborator should be next on the list. The 120 million sales is allegedly for his Chronicles of Narnia series by itself, and doesn't include his numerous non-fiction books or his other novels, such as his Space Trilogy.
5) Anne Rice (100 million)
A surprisingly high number from an author who hasn't produced a truly noteworthy book in some time.
6) Terry Pratchett (55 million)
Up until Rowling overtook him around the turn of the century Pratchett was a bona-fide phenomenon, publishing at least two novels a year for almost twenty years and being responsible for the sales of over 1% of all books sold in the UK and his books hitting the top of the Times bestseller lists like clockwork. Major success in the USA had eluded him until The Amazing Maurice and His Educated Rodents won the Carnegie Prize in 2001. Following on from that, his US profile steadily rose until his books began hitting the NYT bestseller list as well. Aside from the occasional bit of mickey-taking, Pratchett was good-natured about losing out on his position as Fantasy's biggest-selling living author (with the King debate still going on) to Rowling, although his ire was provoked when some Potter fans complained that Equal Rites (1987) ripped off Harry Potter and the Philosopher's Stone (1997), demonstrating a flexible interpretation of causality. Whilst Pratchett has now been firmly overtaken by Rowling, he bore it with equanimity and proudly maintains his position as the UK's most shoplifted author.
7) Robert Jordan (44 million)
Given how it dominates the discussion on some forums, this would seem to be a fairly lowly position for the biggest-selling of the modern epic fantasists. However, by any standards this is a seriously impressive number of books sold, especially given that the sales are split between a relatively small number of books (I suspect his Conan and Fallon novels' sales are all but negligible compared to those of The Wheel of Time sequence).
8) Terry Goodkind (25 million)
Pinning down concrete figures for Goodkind is harder than most due to some truly batty figures being circulated by his fanbase (at one time claiming he was Tor's biggest-selling author but failing to account for why only half as many copies of his latest book had been printed than Robert Jordan's). The worldwide figure of 25 million seems to be well-supported, however.
9) Terry Brooks (21 million)
Recently, with the announcement that movie versions of The Elfstones of Shannara and The Sword of Shannara are in development, it was suggested by some papers that Brooks was the 'second-biggest-selling living fantasy author', which would appear to be hyperbolic. An interview with JIVE Magazine reveals them to be rather more modest, although still extremely impressive. His books have sold very well for more than thirty-one years and Brooks, along with Donaldson, arguably kick-started the entire modern epic fantasy subgenre and has been one of its most reliable and visible writers ever since.
10) Margaret Weis & Tracy Hickman (c. 20 million)
This one was a bit of a guesstimate, coming out of discussions over these two authors' success on a message board several years ago. The figure is certainly highly plausible, with TSR claiming that more than 4 million copies of their Dragonlance Chronicles and Legends trilogies by themselves had been shipped in less than a decade, and this doesn't account for their gaming products, other Dragonlance books and numerous non-Dragonlance novels, many of which have been bestsellers as well.
11) Frank Herbert (18 million)
If there's one thing this list has proven, if you want to be a massive-selling author you're far better writing Fantasy than Science Fiction, unless your SF novel features a ton of Fantasy elements. Frank Herbert's Dune is SF's biggest-selling single novel, with more than 12 million copies by itself sold. I'd also make a fair guess that the other 6 million sales are comprised almost entirely of his other five Dune novels.
12) Diana Gabaldon (17 million)
The author of the Outlander series, in which a 20th Century nurse time-travels back to Jacobite times and falls in love with a Highlander.
13) Eoin Colfer (18 million)
The author of the Artemis Fowl series, which has proven a massive hit amongst YA circles. Colfer was recently picked to write the sixth Hitch-Hiker's Guide to the Galaxy novel, following on from the works of...
14) Douglas Adams (16 million)
...whose exceptionally long periods of writer's block and multiple years spent writing very slim novels, not to mention a poor film adaption of his signature novel, haven't affected his immense popularity.
15) Kevin J. Anderson (16 million)
Whilst his Dune novels co-authored with Brian Herbert may have been critically mauled, that hasn't stopped them selling like hot cakes. When combined with his popular Star Wars and X-Files novels, not to mention original works like the Saga of Seven Suns series, Anderson clearly doesn't have anything to worry about.
16) Raymond E. Feist (15 million)
The author of the extremely long-running Riftwar Cycle of novels, which when complete will comprise approximately thirty books. Mixed reviews for his books published over the last decade or so do not seem to have influenced his legions of loyal fans.
17) Christopher Paolini (12 million)
His Eragon Trilogy (now in four parts) may have been ripped into by the critics with a vengeance, but his popularity is clear. In fact, his sales are all the more impressive considering they are largely based on just two books, with his third only released in the last few weeks.
18) Stephen Donaldson (10 million)
Possibly a surprisingly low showing for Donaldson. His Lord Foul's Bane, published in 1977, kick-started the modern epic fantasy explosion alongside Brooks' Sword of Shannara. However, unlike Brooks who has continued to work in the Shannara universe ever since, Donaldson spent a whole decade trying to stay away from his signature character with works such as Mordant's Need and the superlative Gap series before recently returning to the series, and the bestseller lists, with The Last Chronicles of Thomas Covenant.
19) Neil Gaiman (10 million + )
If GRRM's figure is conservative, this is even moreso, and based solely on the figures I could find for sales of the Sandman graphic novels. Add in his other, highly successful novels and his real sales and position should be much higher.
Edit: Many thanks to Neil for stopping by with some harder figures for this. His figures are 7 million for the Sandman and related graphic novels, plus at least 1 million for Coraline. Factor in his other books and even the above figure looks conservative.
20) George RR Martin (c. 10 million)
Again, another guesstimate based on discussions from various forums and the recent revelation that the Song of Ice and Fire series has sold 2.2 million copies (at least in the USA). GRRM is one of the highest-profile authors in the genre and A Dance with Dragons must be one of the most-discussed unreleased books in genre history. Much to the discontent of those who'd prefer he spent his time on Song of Ice and Fire and nothing else, his recent Wild Cards books have been strong sellers for Tor, and his Dreamsongs retrospective was a significant success as well. I suspect this figure is leaning to the conservative side of things, especially given how big Wild Cards was back in the 1980s.
Update: More information has suggested sales of A Song of Ice and Fire worldwide may be about 7 million by itself, with Wild Cards adding another 2 million or so on top and his stand-alone novels adding hundreds of thousands more.
21) Timothy Zahn (8 million)
Timothy Zahn is the biggest-selling author of Star Wars novel, with the 8 million figure coming from his work in that setting alone (from the Bantam Summer 2011 catalogue). You can add hundreds of thousands more sales from his own novels on top of that.
22) Laurell K. Hamilton (6 million)
Sex sells, obviously, especially when combined with werewolves and vampires.
23) John Ringo (3 million)
The mildly controversial US author ("Oh John Ringo No,") of military science fiction is clearly enjoying the fruits of his success. People may be wondering where his sometimes-collaborator David Weber is, so I direct them to the 'Unplaced' list below'.
Edit: Many thanks also to John for stopping by. His figures suggest closer to 3 million if all his books 'in print' are counted.
24) Harry Turtledove (2.5 million)
The master of alternate history has made this subgenre pretty much his own, with many books about how the American Civil War, World War I and World War II may have unfolded differently. He's also written more overt SF and fantasy works.
25) Peter F. Hamilton (2 million)
The modern lord of space opera has shifted an impressive number of his brick-thick novels and with his US profile now growing rapidly, I suspect he's going to get even bigger in the years to come.
26) Guy Gavriel Kay (2 million)
Thanks to figures researched by Pat from the Fantasy Hotlist, we can add GGK to the list. Given his relatively small number of novels published, this is an impressive figure.
26) Robin Hobb (1 million)
A surprisingly low placing for one of fantasy's highest-profile and most prolific authors? Possibly. This was the figure given by HarperCollins Voyager in 2003 on the completion of her Tawny Man trilogy and applies solely to the nine books published under the Robin Hobb pseudonym in the UK up to that point. They do not include her earlier Megan Lindholm books, nor her later books, nor most importantly her US sales, all of which would likely make her position much higher.
27) David Gemmell (1 million)
Considering how many books he wrote (over 30), this figure may seem a little low. However, Gemmell never entirely cracked the American market, despite being a massive seller in the UK.
28) Steven Erikson (c. 500,000)
This may be even more of a surprise. The original source for the figure was Bantam UK, who announced shifting 250,000 copies of the Malazan Book of the Fallen in the UK in 2006, upon the publication of The Bonehunters. Given another two books have come out since then, and taking into account his Canadian and American sales, a doubling of that figure seems reasonable.
29) Chris Wooding (450,000)
Chris Wooding has been making a name for himself as an adult fantasy author with his Braided Path and Tales of the Ketty Jay series, but his initial success has come from his highly popular YA books, which have sold very well in the United States.
There's obviously a huge number of authors I couldn't find reliable figures for, many of whom would be fairly highly-placed on the list. I'll see if I can't track these down in the future and keep the list updated:
R. Scott Bakker
Iain M. Banks
Frank L. Baum
Orson Scott Card
Arthur C. Clarke
Daniel Handler (aka Lemony Snicket)