Not only do gentlemen read each other's mail; it appears that they read their friends' mail with particular interest. This excellent collection of articles from Intelligence and National Security, the premier journal of intelligence studies, explores how intelligence organizations have kept an eye on their country's best friends. Reflecting the interests of the journal and the availability of documents, more than half of the essays discuss the interwar period and World War II. The book nevertheless carries the story through the Cold War with an essay on the KGB and the East German regime. One recurrent theme is that organizational competition is just as important as national distrust: if a country has two rival intelligence agencies, chances are they will interpret allied behavior differently. A second motif is that reading an ally can be no less difficult than reading an opponent -- and making mistakes can be just as consequential.