Your child is throwing karate chops off the back of the couch and asking to take ninja lessons. This is a shelf for exactly that child. A rhyming fairy tale where the pig who trains hardest wins, a quiet and beautiful ninja-school story set in Japan, the true life of Bruce Lee, the LEGO Ninjago origin, a giggly chapter book about the world's clumsiest ninja, and a tougher summer-training novel from a Navy SEAL. Any single one satisfies the whole thing. Pick by your child's age and by how much of the sport they want, funny, gorgeous, true, or serious.
The thing a parent actually worries about with this shelf is violence, and the honest answer is that these six handle it completely differently, which is the point of a shelf. Two picture books make the training the whole story: The Three Ninja Pigs turns practice into the joke and lets the pig who drilled hardest win, and Little Kunoichi is quiet and lovely, a small ninja who is bad at it until she isn't. Bruce Lee is a true life, a real man who made his own martial art. LEGO Ninjago and Ninja Child are franchise and funny, low stakes and fast laughs. The Way of the Warrior Child is the outlier, a Navy SEAL's book about a bullied boy who gets strong over one summer, with a real philosophy behind it and a military flavor you'll want to know about going in. You reach for the one that fits the child in front of you, and the flavor is usually the fastest way to choose. The one honest thread under all of them: the child asking for this almost always wants to feel capable, not to hurt anyone, and every book here says the same quiet thing, that you get good by working at it.