Even though the Game of Thrones HBO hit is only half way through book three of the not yet finished seven book A Song of Ice and Fire series by George R.R. Martin, don't expect to see future books split into multiple parts. Showrunners David Benioff and David Weiss have told Vanity Fair the current plan is to wrap up the show after “seven or eight” seasons.
Author George R.R. Martin has begun to worry more and more that the show might catch up to his slowly progressing prose. Benioff and Weiss have stated before that they have been told how some of the story wraps up, but last year they were able to sit down with Martin and get more details on how the author plans to end the saga in case show production passes him up.
Last year we went out to Santa Fe for a week to sit down with him [Martin] and just talk through where things are going, because we don’t know if we are going to catch up and where exactly that would be. If you know the ending, then you can lay the groundwork for it. And so we want to know how everything ends. We want to be able to set things up. So we just sat down with him and literally went through every character.
Season Four of the show will (mostly) represent the second half of book three, but may push into book four for some of the storylines. In Martin's series, books four and five actually take place parallel to each other with different groups of characters in each (again - mostly) and began to expand on his already huge cast. Benioff and Weiss may choose to trim in this area (in both characters and story), an area even Martin had originally planned to skip over entirely, leaving seasons five and perhaps six to cover it. That would leave season six/seven to cover book six and seven/eight for book seven. As this is just my own speculation, once they get into plotting out those two parallel storylines for seasons five and six of the show, we'll probably know more.
As for Martin himself, he has long acknowledged everyone's, including his own, worry about the show catching up to him.
I am aware of the TV series moving along behind me like a giant locomotive, and I know I need to lay the track more quickly, perhaps, because the locomotive is soon going to be bearing down on me. The last thing I want is for the TV series to catch up with me. I’ve got a considerable head start, but production is moving faster than I can write. I’m hoping that we’ll finish the story at about the same time…we’ll see.
The five year wait between the last couple novels in the series is what many can't help but think of when they worry about Martin's progress. At least fans of the series, whether of the books or show, will know how the story unfolds within the next four to five years - one way or another.