The show is aleady into Books 4 and 5 for Theon, Brienne and Sansa and Bran. They won't get more than one more season out of books 4 and 5, so GRRM better hurry with Book 6.
As for the Mountain, the books make it clear that Qyburn has him the black cells, he screams in agony for weeks and gets moved to the lowest black cells where nobody will hear him, and his head is sent to Dorne to appease them. But Qyburn keeps taking women that Cersei gives him to "use up" somehow and appears to use them to create headless zombie Gregor as Cersei's new champion (Ser Robert Strong). Ser Robert/zombie Gregor makes an appearance in Book 5 when he carries Cersei in after her naked walk through the city.
If you want to see all the evidence that headless Gregor is now Ser Strong, this essay at the Tower of the Hand site spells it out and includes the relevant book passages (it also covers how the Hound is probably still alive):
http://towerofthehand.com/blog/2010/...he_/index.html