I was hitch-hiking once and the guy who picked me up turned out to be a scriptwriter. We discussed this issue in general and came to the conclusion that it is impossible to make good movies from good books.
Movies require plot, dialog and action, and not too much of even these - there's only 100 minutes or so available. Good books have long descriptions of scenery, feelings, inner dialogues, weather, what someone would have thought if this other thing had happened, etc. How would you film any of that? Good books frequently have little plot. Good books sometimes have very complicated plots that would be impossible to make into a film of less than ten hours in length. Some great books have very little in the way of characters speaking. How do you film irony? Jane Austen can be hilarious in print, but filmed her novels always become vapid boy-meets-girl costume dramas.
Authors know all this, so if they agree to sell the rights, they do not have much cause to complain if the film turns out very different from their vision or is just plain bad. Filmmakers will buy the rights to a blockbuster with the intent to capitalize on the name but in full knowledge that the book is impossible to make a movie of.