The American Black Chamber is a 1931 book by Herbert O. Yardley. The book describes the inner workings of the interwar American governmental cryptography organization called the Black Chamber. The cryptography historian David Kahn called the book "the most famous book on cryptology ever published." By describing the inner workings of the organization, the book created large interest public awareness of the United States's cryptographic abilities. In particular, the Japanese government became aware of the extent of experience the American government had with cryptography and increased the strength of their own knowledge in cryptography in response. Reviewers suggested the book may have cost the United States significantly in the Pacific theater against Japan in World War II.