I think that I have that book in my stacks of what I intend to read, eventually. Despite increasing public access to the primary source material and path-breaking historical accounts, the point remains that even though US cryptanalysis established the existence of Japanese spy and sabotage rings, radio detection and other CI methods had not led to their shut down, even in the more intense security environment after Pearl Harbor. With US war industries and military installations regarded as vulnerable and a Japanese strike at the US West Coast seen as imminent, internment made a lot of sense. I do not see though how internment helped to protect the secret of US cryptanalytic success.
On that last, if we’d have just busted the guys using the codes, the Japanese would have caught on pretty quickly and changed up their codes.