The next installment, The Winds of Winter, in the book series Game of Thrones, will not be released before the new series of Game of Thrones airs in mid-April on HBO.

From George RR Martin's blog - not a blog. Jan. 2nd, 2016 (extract)

Will the show 'spoil' the novels? Maybe. Yes and no. Look, I never thought the series could possibly catch up with the books, but it has. The show moved faster than I anticipated and I moved more slowly. There were other factors too, but that was the main one. Given where we are, inevitably, there will be certain plot twists and reveals in season six of GAME OF THRONES that have not yet happened in the books. For years my readers have been ahead of the viewers. This year, for some things, the reverse will be true. How you want to handle that... hey, that's up to you. Look, I read Andy Weir's novel THE MARTIAN before I saw the movie. But I saw the BBC production of JONATHAN STRANGE AND MR NORRELL before I finally got around to reading Susanna Clarke's novel. In both cases, I loved the book and I loved the adaptation. It does not need to be one or the other. You might prefer one over the other, but you can still enjoy the hell out of both. Of course, there's an aspect to our situation that did not apply to either the Weir or Clarke cases. Those novels were finished before they were optioned, adapted, and filmed. The case of GAME OF THRONES and A SONG OF ICE AND FIRE is perhaps unique. 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. 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.