I echo the unfortunately inactive /u/NientedeNada in recommending The Ninja: An Invented Tradition? by Stephen Turnbull. I've heard some good things about his book Ninja: Unmasking the Myth, but I haven't read it because I'm biased against him from all he wrote before. It might be good if you're looking for something more than a few pages. It might not be. Personally I'm just glad he seem to be correcting what he published before.

The chapter "Autonomy and War in the Sixteenth- Century Iga Region and the Birth of the Ninja Phenomenon" in War and State Building in Medieval Japan is also a good read.

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