A good book that describes the information provided to allied leaders via the intercepts and decryption of the Japanese diplomatic Purple codes is given in the

A good book that describes the information provided to allied leaders via the intercepts and decryption of the Japanese diplomatic Purple codes is given in the book "Marching Orders" by Bruce Lee. ISBN 0-517-57576-0 (c.1995) Lee had access to 1000's of the daily decryption and analysis summary reports (called Magic Summaries,) and provided to the president, Marshall and others. He provides an almost day-by-day chronology showing what the leaders knew and how that knowledge shaped their decisions about the conduct of the war. [edit: the axis decrypts were called Ultra. Allied leaders had access to reports on both.]