To judge a film by the book it's based on is very harsh. Nobody goes to see Verdi's Falstaff and goes home and reads The Merry Wives of Windsor. Nobody berates Verdi for leaving out so much of the play. And that should apply to film as well. When you adapt a book for film, you lose 60 or 70% of it.
The cinema is a world of constraints and parameters and barriers, while the world of writing a novel is freedom.
[1991] If you don't like collaborating, then don't write screenplays.