Aragorn and the humans don't figure into the books as prominently. The movie made him into a sort of chivalric Knight of the Round Table co-star but the books are told mostly from the perspective of Frodo, Sam, Merry and Pippin as the view this grander battle going on around them. The films placed more emphasis on the battles and action and so the side story of Aragorn was brought more to the foreground as a sort of co-star to the final two films. Also, the first half of Frodo and Sam's story, up until Frodo's capture after being stung by the Shelob, occurs in The Two Towers, and after the final battle where Sauron is vanquished, teh Hobbits return to the Shire to discover it has been torn apart and is under control of Sauromon, whom is finally killed. Jackson actually changed quite a bit but in my opinion it was defintily for the better (especially the anti-climatic "scouring of the Shire" section, with its Scooby Doo-like reveal of Sauromon).