It's rare that they make a good film out of a book. Rarer still they make a great film out of a book.
Usually, they take a great book, and make a "Long and boring" film out of it. Or three of them. Think Peter Jackson's "Lord of the Rings.". Or Harry Potter.
Or, in the case of the "Titanic", they don't even use a book, just a scrap of toilet paper the director made some notes on, maybe an old Reader's Digest to do their fact checking....
"Fight Club" is a great book. And it was a great film. And, oddly enough, (perhaps because I saw the film first), the one didin't ruin the other. Yes, they were close enough, but there were enough differences that you didn't feel you were simply seeing the film again. There were twists, director's interpretations, things in the film that were changed to make it tighter....
But the book is good too. Great even. It's a wonderful rant, a spit and jab at the world of commerce and social expectation, an anarchist's Bible. A well reasoned, well written invective on the futility and emptiness of modern life. A sort of real life self help book.