"The Kouga Ninja Scrolls".
A historical fantasy novel by Futaro Yamada. Aside from the manga Basilisk which is an adaptation, the iconic 90's anime film Ninja Scroll is also loosely based on it. That's not even getting into how influential in general this book was to the depiction of ninja, even things that are tonally completely different like Naruto owe a lot of inspiration to this book.
Needless to say, I am very interested to dive into this and see if the book lives up to its reputation.