Winds of Winter, the next book in the A Song of Ice and Fire series upon which the HBO drama is based, holds key answers like whether Jon Snow (Kit Harington) died at the hands of Night's Watch or how Daenerys Targaryen (Emilia Clarke) fared with the Dothraki.
"The Winds of Winter is not finished," Martin wrote. "Believe me, it gave me no pleasure to type those words. You're disappointed, and you're not alone. My editors and publishers are disappointed, HBO is disappointed, my agents and foreign publishers and translators are disappointed...but no one could possibly be more disappointed than me."
Martin says he's not sure when the sixth book will be completed, just that "It will be done when it's done." However, the note suggests that though the book won't be out before Season 6 debuts, that doesn't mean the series will contain book spoilers.
"I can't think of any other instance where the movie or TV show came out as the source material was still being written. So when you ask me, 'will the show spoil the books,' all I can do is say, 'yes and no,' and mumble once again about the butterfly effect," Martin says. "Those pretty little butterflies have grown into mighty dragons. Some of the 'spoilers' you may encounter in season six may not be spoilers at all... because the show and the books have diverged, and will continue to do so."
There had been hope when HBO's spring and winter dates were announced that Game of Thrones being delayed a few weeks later than normal meant the network was stalling for Martin to have the book released ahead of the season. That could have been the case if Martin had met his October or December 2015 deadlines, but he missed both of them.
Game of Thrones Season 6 is set to premiere in April.