I want to see the movie in the book. If it doesn't have a good ending I'll pass because I'm not sure I can come up with a great one. Adaptations are much, much easier and you're more likely to get the picture made. A story you sold on a pitch may turn out differently than the studio thought it would, while the story in a book is clearly known by all from the beginning. And books that have great characters give you a huge leg up, because you don't have to figure out who your characters are going to be.
Short stories are the best to adapt.
They're complete in that they have a beginning, middle and end, but they are short enough that you get to fill in instead of having to cut out so much. I read the book, then let two weeks pass and see what stayed with me. Whatever resonates is what goes into the script. I'm aware that the kind of satisfaction I get from a reading experience is entirely different than with film. Some things you enjoyed reading about will just stop your movie dead.