"Mathematical Cranks" by Dudley Underwood.
Actually, I started feeling a bit bad about laughing at some of these people who are clearly mentally unbalanced. Dudley Underwood was sued recently by one of the people mentioned in his book. I don't know the details but apparently the person was arguing that Underwood has misreprented what he had written. If I recall correctly the court ruled for the defendant on the grounds that "crank" was not well defined enough to be libel.