I think the most crucial (and often overlooked) difference between the films and the books is the narrator's perspective. The films are largely shown from a neutral / omniscient narrator's perspective. The books OTOH are mostly written from the perspective of the Hobbits. (The conceit is that the books are adapted from written hobbit lore collected mostly by Bilbo and Frodo[1].) The implication is that there's much more emphasis in the books on what's important and interesting to the Hobbits, as opposed to Men or Elves. In the film, Frodo and Sam are only arguably the main characters, and only because they are the Ringbearer and those that destroy the Ring in the end. But the boook is really Frodo's Own Story.

[1] https://en.wikipedia.org/wiki/Red_Book_of_Westmarch