Aapo E.
Star Wars: Episode 3: Revenge of The Sith. I felt like it dived deeper into the characters and especially in the grand finale, Order 66. It brought up the differences between the sith and the jedi, and the different goals of both sides, and methods for accomplishing them. I understood the relationship between the jedi and the sith better through this book.
Tri A.
All of them XD From Harry Potter, The Hobbit and all of Dan Brown's books.
I have to admit though that certain movies contribute to the visualisation of the intricate book descriptions out there. Peter Jackson's adaptation of The Lord of the Rings is filled with shots of the majestic landscape of Middle Earth. While reading the books in parallel, I found the plot more immersive than ever and my love of Tolkien only keeps on growing.
Lucas E.
I would say The Outsiders. I really loved the book, the movie was great too but I always find the books the best. There are still movies adapted from books that I haven't seen though. Such as The Fault in Our Stars and All the Bright Places.