While Game of Thrones has deviated from George R. R. Martin’s source material repeatedly, most notably in the recent fifth season, the books have generally still been a reliable guideline for where the story is headed in broad strokes. According to one cast member though, the actors on the show are generally discouraged from reading the books.
Iain Glen, who plays Jorah Mormont, told Radio Times that he hasn’t read any of the books other than the first and that showrunners D.B. Weiss and David Benioff don’t really want him to anyway:
“I don’t read the books. I read the first one when they gave me the job, just to get a feel from it and a synopsis breakdown of what was going to happen to the character. But beyond that, the writers themselves, David and Dan, they didn’t particularly want actors coming to the scripts from the book, always suggesting what the book did and how it was different…They were quite happy for us to move forward and treat them as screenplays with no history, so that’s the reason why I only ever read the first novel.”
Considering that Martin currently has no new book to glean answers for season five’s dramatic cliffhanger from, there’s really no danger of any of the actors reading ahead right now. Glen also had a bit to say about how he’d like to see Jorah’s story wrapped up: “I just want Jorah to be there when Daenerys finally succeeds in the end, and sits on the Iron Throne. I’d like to be beside her.”
Given Jorah’s current stoneman infection (itself a dramatic change from Martin’s books), the elder swordsman’s chances of making it to the end seem pretty slim.