If you're like me, then you looked at that title and wondered what the heck we were getting into. It sounds like a legitimate horror story, and I wouldn't have gotten within ten yards of it if I hadn't also seen the illustration on the cover.
First, this is definitely a kids' book, and second, those kids look very anime, which is usually a good sign. :) So yeah, I [picked it up and gave it a listen, and I think that was a pretty good call.
There's a genre, the name of which I simply do not know, wherein almost every story begins with the main character being shipped off to spend the summer with relatives. Usually the main character is not pleased, or at least confused, and spends the first chapter or two asking questions like "Why am I going here alone?" and "What am I supposed to do while I'm here?"
Some of these books go into a self-discovery theme, where the main character learns from the area, their relatives, or informative townspeople about their heritage and "who they really are." Others take the dive into fantasy with "magic is real" and "you're a secret guardian" plots.
I'm happy to give this book a solid 4 out of 5 for vivid characters, decent worldbuilding, and fun descriptions. There were some moments in the book that jarred me out of the story, wondering either why the kid didn't think of X sooner or how the kid mastered X skill so quickly, but those were pretty minor bumps in the road.
Oh, and extra credit for the twist near the end, which caught me by surprise. I'm usually pretty good at seeing those things coming, simply because tropes are easy to read, but I didn't start suspecting this one until it was almost on top of me. Two thumbs up.
I'd say that if you like books like Misty of Chincoteague for independent kid action or Harry Potter for the "OMG magic is real" plot of the first book, then this is probably a good bet for you.
It probably goes without saying, but I would warn you against this book if you don't like teeth. There are a lot of them in this story.