What makes a book a candidate for a good film?
Answer: it depends.
On many things.
Sometimes, it's the visuals; the story is quite cinematic, begging for a visual representation.
Blade Runner certainly comes to mind.
Also, a thoroughly-visualized world (one perhaps not recently seen) is important. Think Harry Potter (the novel manuscript of which, you'll recall, was roundly rejected by all publishers for over a year; oops).
For other books, it's not about the visuals so much as it's about the story, the theme, the message, whether it be serious, quirky, or otherwise. Cohen Brothers films come to mind. You always know you're going to get something wacky yet thought-provoking with them.
No Country for Old Men, for example; which, interestingly, began as a screenplay, became a novel, and then became a movie; so, there are no rules; like William Goldman said: If it works, it's right.
And, finally, it is very much that unknown element which is an almost-mystical combination of cultural and strategic business synchronicities. The process of a book being written, edited, published, adapted, and the film version being produced, with the thousands of people involved (along with a lot of financial investment) is such a massive process, it's a wonder books are published and films are produced.
Yet, despite the significant resources required to do so, both are still art. And art is subjective. That, I believe, is the majority of our answer.