I've read the books and I found the historical sections very interesting. I must say he took very few facts and ran with them. He could be right, the idea that Japan is a mix of two migrating groups really caught my imagination. That said I would be very careful to quote his supposition as fact.
Originally posted by paw
Tetsutaka Sugawara has a series of books on this very topic. "Aikido and Chinese Martial Arts". I believe two volumes have been released and three are planned (someone please correct me if I'm wrong).
To the best of my knowledge, Sugawara Sensei travels once a year to the United States to teach. If you are interested, I can see when and where he will be in the US. (Any Katori Shinto Ryu folks lurking with better information?)