Underwook Dudley — Mathematical Cranks — The American Mathematical Association, 1992, 372 pp. ISBN 0-88385-507-0
Everyone using the Internet knows about cranks. A typical crank is an individual putting forth a defective theory, usually mathematical or physical in nature, but refusing to see his errors or understanding what he missed; despite well intentioned explanations from (real) experts on the topic. The crank insists with denial and eventually aggressivity.
Dudley presents a number of crank theories and explain how and why they are defective by explaining the real mathematics behind the theories concerned. But more interestingly, he exposes the psychology of the typical crank and how a mere amateur mathematician transforms himself into a crank. Some of the stories and chapters are a bit too long, but nonetheless, I had a few good laughs; definitively worth reading!
The Google Books excerpt of the book.