Well, the Great Gatsby was already made into an opera that premiered at the Met a few years ago, but it would also make a good musical I would think.
As for other books that would make a good musical, it's always hard to tell. Books when read hardly seem like they can sing right off the page, and clearly they were not originally intended to do so. When you read Ragtime by Doctrow, it doesn't quite scream MUSICAL!!! but when you see the show, you understand how it works out.
Yes, there are some books that seem to lend themselves easily to musicalization, but it's really the matter of how the creators are able to take a pre-existing story, and create a compelling piece of work that's their own, but also pays respect to the original author. This in itself is very difficult.
-There's the muddle in the middle. There's the puddle where the poodle did the piddle."