It came down directly from George R.R. Martin, the king himself, that Book 6 of A Song of Ice and Fire, The Winds of Winter, will not be complete before the HBO series this spring.
What does this mean for fans? You can read Martin’s blog post on the subject here. The short version is basically this. He’s not done with book because he’s a very busy guy. He travels to promote both the HBO and book series, and frankly, a story like A Song of Ice Fire doesn’t just fall out of the sky on a whim. As far as spoilers in the show, there will probably be some. There will also be things in the book that will never make it to the show, so according to Martin, The Winds of Winter will still be worth reading even if you watch the show.
I feel for George, there’s a ton of pressure on him from all directions to write more Game of Thrones content. Also, I would much rather have a completed book that Martin is proud of rather than something just pushed out for a deadline. George R.R. Martin, you take your time and write a another great book!