According to this link it seems that George RR Martin is pushing his hardest to get the next book out before Season 6 is broadcast. It is quite obvious that one of two things happened in the production of Game of Thrones – 1. That HBO didn’t expect it to be so popular and therefore they would never have to worry about how the series would end or (more likely) 2. With the time it would take to get to Season 6, that the producers expected that the book series would have been far more advanced than it is at this stage.
It will be interesting to see how Season 6 will be received should the book not be released. The show runners have stated that the show will go in different directions to the books, however this won’t be absolute. Will the readers of the books wait to watch the series, or will they watch anyway and hope to get some more world and character building when they eventually get the book in their hands?