Game of Thrones showrunners David Benioff and D.B. Weiss have confirmed that the series will inevitably overtake the published books by George R.R Martin. Speaking at the Oxford Union, co-showrunner David Benioff commented on the tricky situation they are facing.

“We've been talking to [Martin] for a while about this. The fact is, we didn't want to catch up. Obviously, we would hope that the books would come out ahead of the show. But at the same time, George has his process. And if it takes him 20 years to finish the series, that's what it should take him. He's writing, to my mind, the great fantasy epic of our time. So we can't rush him, and I wouldn't want to rush him. [But] at the same time, we can't put the show on hiatus. ...

So we have to kind of keep pushing forward. Luckily, we've been talking about this with George for a long time, ever since we saw this could happen, and we know where things are heading. So we'll eventually basically meet up at pretty much the same place where George is going [in "A Dream of Spring"]. There might be a few deviations along the route, but we're heading towards the same destination. I kind of wish, there were some things we didn't have to spoil in terms of the books, but we're stuck between a rock and a hard place. So the show must go on. ...”

Martin has been working on The Winds of Winter since the previous book (A Dance With Dragons) was published in 2011. Prior to that, there had been a five-year gap between the previous book, so worse case scenario it could be as late as 2021 before the last instalment hits the shelves. The forthcoming fifth season draws from the fourth and fifth books, this means next year’s sixth season will move towards the events of The Winds of Winter. As Martin has divulged the ending of his fantasy creation to the showrunners, it is entirely possible that the series will complete Martin’s story before he does.
Earlier this year Martin announced that the long-awaited The Winds of Winter was unlikely to be released in 2015. In an effort to complete the eagerly awaited penultimate tale, Martin has cancelled his planned appearance at Comic Con as part of the Game of Thrones panel. In an ideal world, The Winds of Winter would come out just before Christmas this year with the seventh and final book A Dream of Spring out by 2017. A geek can dream.

Game of Thrones returns to HBO on April 12.