Mary Pope Osborne 
Magic Tree House 5: Night of the Ninjas 

Eight-year-old Jack and his little sister, Annie, are playing in the woods during their summer holiday, when they find a mysterious tree house full of books. But these are no ordinary books . . . And this is no ordinary tree house . . .
Morgan le Fay — the owner of the Magic Tree House and all its mysterious books is missing and needs Jack and Annie»s help! They follow her clues to Japan where they find themselves in the middle of a feud between Japanese ninja warriors and Samurai warlords. Will Jack and Annie find the tree house again? Or will they be caught up in a terrifying battle?