Very often, when a book is turned into a movie, the result turns out not that good. The producers are always struggling with squeezing a life story into an one and a half, two hour movie. How are they supposed to do that successfully? That is why people often say "oh, the book was so much better than the movie", because the book is always richer with info and events.
As good as, for example, the Donnie Brasco movie is, the book (at least the 1986 edition) is so much better. There is a lot of good stuff in the book, that was never described in the movie.

Another thing that is quite irritating (at least to me) is when the same actor appears as the same character throughout the movie, even if the character has aged 40, 50 years.
So easy to see all that make up and not many 20 year old actors can act the character of a 70 year old. And vice versa!