So. I enjoyed this because it harks back to a kind of classic fantasy, for which I have a considerable soft spot. Sometimes a classic tale is classic for good reason. This is the basic structure of a chivalric romance or hero's tale and its been thriving for well over a thousand years because people just like it. Some basically good hearted companions go on a quest for a noble cause and are aided or hindered by those they meet, often including creatures out of folklore. King Arthur and the Knights of the Round Table, Orlando Furioso, Roland and the Matter of France, Don Quixote, The Three Musketeers.
Or in children's literature, Narnia, The Hobbit, Five Children and It, The Never Ending Story, The Homeward Bounders, and on and on. This book belongs to a long and noble tradition. In this case the folklore is that of Japan, and the companions are a family of children in search of their little sister. Like many other books of this type this one doesn't waste a lot of time on back story or on trying to explain the logic of the situation or the subtle psychological undercurrents of the characters' motivations. It just leaps in with both feet to an adventure already in progress and off we go at a run.
I think with the recent rise in the popularity of ya fantasy some of it has gotten a little over written. This harks back to an older tradition, one that lets the adventure and the folklore take center stage. The author clearly knows his Japanese folklore but he doesn't leap in to explain to us at great length what an oni is, he just lets it chase the kids through the garden and lets us pick up what it is like as we go along. I really like that because I think any folklore with a long tradition has enough resonance to carry some weight on its own. I think the story is better because he lets it do so.