George R.R. Martin has reported that the writing part of The World of Ice and Fire has been completed, with the book handed over to the editors for the last artwork to be added. Assuming no problems with the art, GRRM anticipates the book being published in October this year.
The book will feature a detailed history of Westeros (particularly focusing on the Targaryen era) as well as geographical information. Originally the book did not feature much of Essos and the east, but at popular request the writers have added more detail to those sections.
Expanding on GRRM's information over at Westeros.org, co-author Elio 'Ran' Garcia confirms that the bulk of the work was done last year and most of GRRM's writing time since has been dedicated to The Winds of Winter, returning to finish off a couple of last pieces for the world book before the copy-edit.
The book will be over 150,000 words in length with well over 200 illustrations, the majority new (although Ted Naismith's castle artwork from a calendar a couple of years ago will be reused, as will some material from the card game). The book will probably come in between 300 and 400 large-format pages in length.
Work on The World of Ice and Fire began way back in 2007, so it'll be good to finally see it hit the shelves and hopefully clear up a lot of questions about the world of Westeros and Essos.