The book recommended to me when I was doing Ninjutsu was "Ninjutsu: History and Tradition" [1]. It's written by Masaaki Hatsumi himself.

[1] http://www.amazon.co.uk/Ninjutsu-History-Tradition-Masaaki-H...

Chozanshi's Demon's Sermon on the Martial Arts translated by William Scott William. My friends have found that book awesome. Hell, so have I, and I'm not training in the lineage. Chances are, that book is gooblygook to you and you're better off with something gentler like Dan Millman's Way of the Peaceful Warrior instead.

I'm being a bit of a trickster here. @samsteve has some direct sources.

Oddly enough, my first thought was to suggest talking to your local public librarian. See if they can help you find some good books on the subject?