The Washington Post called this book "laughably ignorant," but it's a delightful read. Conspiracy buffs will love it, especially those who believe in a Vast Right Wing Conspiracy of Republicans bent on twisting history to their own money-grubbing advantage.
The history of the Meiji, Taisho, and Showa emperors up to 1945 isn't bad, as opposed what follows. The Seagraves have a knack for making individuals and situations come alive. They also have a knack for getting things wrong: MacArthur escaped from Corregidor by PT boat, not submarine; Japan had army and navy air forces, not a distinct "Japanese Air Force"; the great fire raid on Tokyo featured incendiary bombs, not napalm, and it killed about half the 200,000 cited by the Seagraves; in 1948 Edward Lansdale was a major, not a general....
More ominously, for a book that purports to give the inside scoop on the Emperor System, the Seagraves don't read Japanese and rarely if ever had translations made. For the first half of the book, I read the copious notes along with the text, and found no instance in which the Seagraves refer to a Japanese text. I can't be sure of this because I gave up this practice when I realized that the really interesting stuff was never supported by a source I knew and trusted.
Golden Lily, for example: as the Seagraves tell the story, Japan looted the nations it conquered, hid the treasure in caves in caves and sunken ships, and used it to enrich the emperor, bribe MacArthur and Herbert Hoover, finance the country's postwar expansion, and fund the U.S. Central Intelligence Agency. Evidently the Seagraves came across some (uncited) informant, then spun a book around this germ of a story, using whatever English-language sources they could find.
Read it by all means, but don't take it too seriously.