Night of the Ninjas by Mary Pope Osborne

The tree house takes Jack and Annie to ancient Japan, where they meet mysterious ninjas who guide them through a dense forest. The siblings must be quiet, cautious, and clever to navigate the unfamiliar terrain and complete a riddle. Along the way, they learn the values of focus and inner strength. Their journey is filled with stealth, suspense, and cultural discovery. It’s a calm yet thrilling tale about trust and wisdom.