The books are concerned with the adventures of Geralt of Rivia, a "witcher" or monster-hunter. As a mutant, he has superior reflexes and can imbibe potions and use oils to gain the edge in combat. His day job is killing the various monsters which inhabit the Northern Kingdoms, but over time he gets reluctantly drawn into politics and a major war brewing between the kingdoms and a powerful empire to the south, not to mention racial struggles between humans, dwarves and the elves they have hunted to the edge of extinction and are now fighting back.
There are eight books in The Witcher series: Sword of Destiny (1992), The Last Wish (1993), Blood of Elves (1994), Time of Contempt (1995), Baptism of Fire (1996), The Tower of Swallows (1997), Lady of the Lake (1999) and Season of Storms (2013). They have sold over 2 million copies in 20 languages since release, with the books being particularly popular in Poland, Russia and Spain.
However, far better-known to Western audiences are the three Witcher video games: The Witcher (2007), The Witcher II: Assassin of Kings (2011) and The Witcher III: Wild Hunt (2015). The games have sold over 20 million copies in total, with The Witcher III being lavishly praised as the best CRPG of the current generation.
It is unclear if the TV show will adapt the books or will pursue original stories set in the same world. This announcement does seem to decrease the likelihood of Netflix being the home of the upcoming Wheel of Time TV series, since it is less likely that Netflix will want to develop two medieval fantasy TV shows simultaneously.