Few authors have so successfully mined the history of codebreaking. In Kahn's latest book, he explores the life and times of Herbert Yardley, one of the forgotten figures of U.S. intelligence. Yardley's achievements had less to do with his talents as a cryptanalyst, which were modest, than with his ability to mobilize U.S. resources during World War I (and for some time afterward, including for the successful effort to break Japanese codes during the 1921 Washington Naval Conference). His career took a dramatic turn after his unit was closed down in 1929, following Secretary of State Henry Stimson's famous observation that "gentlemen do not read each other's mail." Yardley, facing destitution, decided to tell his story in a best-selling memoir, The American Black Chamber, which introduced the history and methods of codebreaking to a wide audience. The book turned Yardley into a minor celebrity but also earned him the ire of those who felt he had disclosed too much about matters of national security. For this reason, his attempt to return to service during the next war -- this time via Canada -- failed. Kahn's book includes more than one needs to know about Yardley, but it is at least an entertaining read.