I've been invited to give a talk about books and faith and naturally I have a few favorites I'd like to discuss with the folks who come. What has become increasingly apparent to me is that many books, especially fiction, seek to explore what the spiritual world might be. In the process they often help to redefine what we understand about spirituality, and therefore about our faith in it (or lack thereof).
Perhaps one could go even further: any book that is any good will inevitably ask difficult questions, even questions that have no answers - and so to some extent will push the boundaries of existing understandings about faith.
Literature, in general, seeks to show through words that which cannot actually be stated in words. That's why poetry evokes far more than it defines. That's why a passage of Jane Austen suggests more than it says. We are asked to be part of the process of making meaning - and each one of us will make a slightly different meaning.
Perhaps one could say that all writing, all books, question faith. In the process of doing so they may actually help to build that faith into something stronger than it was before.