One of the first major pieces of ninja fiction I am aware of is a novel by suspense novelist Eric Van Lustbader, simply titled The Ninja. Published in 1980 it combines martial arts intrigue with a ton of international intrigue.
Nicholas Linnear is born in Japan during the days after World War II to an American general and his Chinese wife. He grows up in Japanese schools and quickly becomes adept in a variety of martial skills: aikido, kenjutsu, and eventually “white” ninjutsu. He comes in to conflict with a cousin, Saigo, who despises him in every way. He also falls in love with a young woman named Yukio (very possibly the inspiration for the later X-Men character of the same name). Saigo will do anything for vengeance on his cousin and begins the study of black ninjutsu.
But things end terribly in the past for Linnear. He eventually flees Japan, alone, soiled and virtually destroyed by Saigo.
In the modern day, he is living comfortably and living a peaceful life in America. That life ends as a businessman dies, apparently at the hands of a ninja. A detective named Lew Croaker latches on to Nicholas Linnear as an aide in the investigation. It doesn’t take long for Linnear to realize the culprit is his old foe. He must overcome his own memories of the horrors Saigo put him and Yukio through before he can hope to stop the merciless killer.
A vein of international intrigue runs through The Ninja. That vein would burst in to the plots for five more books in the series, all sadly drifting farther away from Linnear’s ninja roots. A warning to readers: The Ninja graphically depicts its sex scenes, including a brutal rape, although later books in the franchise go much, much farther. Still this novel represents its genre well, digging deep in to Buddhism and Shintoism to present as realistic a portrayal of Japan and the ninja as could ever be possible upon its release. Recommended.