I have not seen the film, but I have read the book. I understand there are some minor differences between the book and the film. At any rate, I'd suggest picking up the book if you are interested. It is based on a true story and pretty well researched, though the author invents and embellishes where needed to fill in gaps. So the events are "true" in that they definitely could have happened, and they don't contradict other elements of his life or character, but not true in that they are simply fabricated. In his own words "The Revenant is a work of fiction. That said, I endeavored to stay true to history in the main events of the story. A specific example would be that Glass did catch up with Fitzgerald eventually, but what transpired between them is unknown, so Punke made up something compelling which did not contradict later events.

Punke also lists most of his sources in the back of the book. Many of them are just survival guides which he used to supplement other information to keep the story believable. Of the books used, Stephen Ambrose's Undaunted Courage is very readable and a great introduction to the exploration of the Louisiana Territory through the exploits of the well known Louis and Clark. John Myers Myers' The Saga of Hugh Glass and Stanley Vestal's Jim Bridger, Mountain Man are the biographies of two of the main characters in the film and book. I'd add that Saga only covers the early part of Glass' life, so you could pick up the much older The Song of Hugh Glass for more of his life, or the more contemporary Lord Grizzly. Jim Bridger is a pretty famous person in the old west, so there is plenty written about him.

Hiram Martin Chittenden's The American Fur Trade of the Far West is long and quite old, but incredibly well researched. I'd put this one off until last if you are interested still. It is old enough you can probably find it online for free with no copyright.For books on early America, I'd recommend Empire of Liberty and What Hath God Wrought. It is hard to go wrong with anything from the Oxford History of America.

If I may pose an addendum to the question: how specifically did the Pawnee people interact with white settlers. Are the novel/films depictions of trade and interactions (including sexual) accurate or and or commonplace?