For around 15 years I accompanied the 'last Ninja grandmaster', Hatsumi Masaaki to his seminars around the world, as his interpreter. Later I also translated his best-selling book, the Way of the Ninja (see below).
Ninjutsu is not well understood, despite (or maybe because of) the huge number of books written about it. Until a truly 'definitive' work appears (one of these
days years), I suggest that those interested read a selection of the works listed below.

The links in each case are to Amazon.