This free download book arises from the movement over in the UK which is attempting to destigmatize those who hear voices. While the language isn't perfect, it is a decent "harm reduction," client-centered intro to why and how a therapist can work with clients who experience things the therapist is unable to verify.
Beyond Belief - Alternative Ways of Working with Delusions, Obsessions and Unusual Experiences
http://www.peter-lehmann-publishing.com/beyond-belief.htm