You Don't Have to Be Crazy to Hear Voices...

... although, no doubt, it helps.
But that cheap joke aside, Daniel B. Smith, who was on The Colbert Report last night, has written what looks like a fascinating book on the phenomenon of auditory hallucination through the ages, centered around the story of his own father, an otherwise normally functioning attorney who heard voices throughout his life.

Muses, Madmen, and Prophets: Rethinking the History, Science, and Meaning of Auditory Hallucination