From the introduction: PHBR15: The Complete Ninja's Handbook is a book of supplemental rules for adding depth, spice, and life to the ninja. However, even if your DM should decide that none of the [additional] rules within this book can be used in his campaign, you can still gain a lot by reading the pages that follow. It is also possible mimic most of the ninja kits within the existing thief class. This tome answers many questions about ninja abilities and presents an extensive list of weapons and gimmicks available to the ninja. And, finally, this book has lots of information on role playing ninja characters.