I bought this book for one of my brothers for Christmas as a joke, and we had a lot of fun laughing at it. Ashida Kim (who's real name is Chris Hunter) seems to be lost in a 1980's ninja movie inspired fantasy world. You'd think he learned ninjutsu by watching Sho Kosugi movies.
Some of the "ancient secrets of surprise" in this book include tapping a sentry on the shoulder then moving in the other direction when he turns his head to look, and jumping out from behind trees and bushes. Heh heh heh.
If you've got money to waste and are up for some cheap laughs, then any of Ashida Kim's books will do. If, however, you are looking for books on real-world martial arts, get something else.