Moonshadow: Rise of the Ninja by Simon Higgins
Moonshadow is a ninja in training. He works for the Grey Light Order and they have decided he is ready to go on a mission. Disguised, Moonshadow travels to the fortress of a war lord to steal his newly aquired plans. When he gets there, he is not only confronted by the guards, but another ninja looking for the plans as well. Will Moonshadow complete his mission successfully, or will he fail? And who is the mysterious other ninja?