Magic Tree House: Night of the Ninja, Book 5 by Mary Pope Osborne

Jack and Annie are ready for their next fantasy adventure in the bestselling middle-grade series?the Magic Tree House! Have you ever met a real live ninja? Jack and Annie do when the Magic Tree House whisks them back to ancient Japan, where they find themselves in the cave of a ninja master. Will they learn the secrets of the ninja? Or will the evil samurai warriors get them first?