Shawshank Redemption is a great film not because it is close to the book but because it is a great adaption.
In the book, there are 3 wardens. In the film, there is only 1 and it is better for it.
In the book, Boz gets beat up because Andy arranges it. In the film, the warden is responsible and Andy comes across as a more sympathetic character.
In the book, Red is a red-headed Irish man. While that might have worked, it is impossible to imagine anyone other than Morgan Freeman in the role.
I don't think a film has to stay true to a book.
Having said that, I did enjoy "Sherlock Holmes goes to Washington" with Basil Rathbone being based on "The Dancing Men" because of one "blink and you'll miss it" scene in the film.