He has been working on the sixth book (The Winds of Winter) for over eight years! Martin is closing in on 71....not saying that's old, but there is also a seventh planned to finalize everything (A Dream of Spring).
He said in an older interview: "It’s not the end for me. I’m still deeply in it. I better live a long time because I have a lot of work left to do."
In 2017, the writers for GOT mentioned having prequel spin-offs which I'm looking forward to. I remember when the same was done for Breaking Bad (to Better Call Saul) and I wasn't disappointed
I'm just wondering how much the books will differ from this last GOT TV season.