Book excerpt: What constitutes historical truth is often subject to change. Joe Nickell demonstrates the techniques used in solving some of the world's most perplexing mysteries, such as the authenticity of Abraham Lincoln's celebrated Bixby letter, the 1913 disappearance of writer and journalist Ambrose Bierce, and the apparent real-life model for a mysterious character in a novel by Nathaniel Hawthorne. Nickell also uses newly uncovered evidence to further investigate the identity of the Nazi war criminal known as ""Ivan the Terrible.""