Just finished reading The Revenant. It's a fascinating story which supposedly is mostly true with some dramatic flair the author admittedly uses to fill in the gaps that history doesn't tell us.
The ending is partial fiction, at least his final encounter which in the book is very anti-climatic. The survival part being driven by vengeance is true and most of the indian encounters are generally regarded as having actually happened.
It takes place in an area and time where there isn't a lot of recording of history going on beyond first hand cartography Most of what is recorded comes from first or second hand stories being told at forts and outposts by the trappers who first attempted to tame that land, shortly after the Lewis and Clark expedition, and sometime sought shelter, provision or even trading at the Army posts. There are letters and dispatches from trappers back to their investors in St. Louis that helps corroborate the stories. I can't imagine the toughness it took for any of those men to go there at that time much less the toughness it took for Hugh Glass to survive. It must have taken tons of research to pull it all together and get the book historically accurate.
I'm definitely looking forward to the movie.