I know a published author who was approached by LifeTime about turning one of his novels into a teleplay. The network wanted to make some changes that would have altered the location and updated the time period, among other things. He considered their offer, then declined with the phrase, “No. It would ruin my book.” What exactly does that mean?
A book exists whether or not they make a movie based on it. It has substance and can be purchased exactly as you wrote it. The book still exists unaltered even if you sell the movie rights to someone who changes the story and characters around. Ultimately, a book stands on its own, regardless of derivative treatments. The book is the book. A movie based on the book is a movie. These two things do not necessarily overlap.
I enjoy reading the source material for movies I like. Recently I read The Authentic Death of Hendry Jones, on which the great Marlon Brando vehicle One-Eyed Jacks was based. These two treatments were poles apart. With the exception of some character names, locales, and one or two scenes, they had nothing in common. Evidently, the movie producer, screen writer, and director (Brando) had a vision quite different than the book author (Charles Neider). Yet I enjoyed both. More importantly, both existed and were entertaining despite the fact that they were quite different.
In my opinion, authors (especially little-known authors) shoot themselves in the foot by this pompous attitude. A movie or teleplay treatment of your book does not alter the book, so it is not possible to ruin it. What it does is provide additional monies and publicity for your book. Even if the movie treatment is lousy, your book still benefits from a new cover with something like “Now a major motion picture” on it.
Win! Win! Win!