The Magicians on SyFy diverged quite radically from Lev Grossman's trilogy, ultimately building a distinct and significantly larger mythology and universe. I loved both the books and the first three seasons of the show, while realizing each is its own thing. The fourth season of the show, though, was terrible.
I liked P.T. Anderson's movie of Inherent Vice even better than Pynchon's novel. Joaquin Phoenix was amazing in bringing the lead character to life, as was Katherine Waterston, especially in that scene (if you've seen the movie, you know which one). I like the movie ending better than the book one.
Nicole Holofcener's film of Ted Thompson's The Land of Steady Habits (available on Netflix). Everybody in the movie is about ten years younger than their counterpart in the book, and I think that works much better. It's kind of strange in the book when people are shocked that the lead character took early retirement, though he's in his early 60s. (Not that much of a stretch, folks.) Now, early 50s, on the other hand... And his son, who just can't get his life together, works better as someone in his mid 20s rather than the book's 33.