Foremost, Turnbull writes for a popular audience, but he publishes with reputable, major publishers. His books typically have bibliographies and often endnotes. There is deeper academic work around, but he's a good place to start. You might also be interested in Kacem Zoughari's book on Ninutsu. He is a Ph.D in the Japanese history and culture. BUT, he's also a Bujinkan megadan, and he glosses over any connecting threads between the history and his account of Takamatsu.
Thank you for telling me about Stephen Turnbull. I'm looking him up in www.amazon.com and it appears to me that this is sound, academic material.