As is often the case with Gaiman, this book is a mixture of fantasy and horror, though the reader is invited to decide what the proportions are. The short review is that if you like Gaiman, you'll probably like this, and if you don't, then this won't change your mind.

The story tells of an unnamed seven-year-old boy and the events that happen to him, revealing things about the true nature of reality in the process. Yes, it has Wise Old Figures and Things Man Was Not Meant To Know and Your Reality Is Part Of A Deeper Reality, and a bunch of other tropes you may have seen before, but that's like saying that a movie has action scenes and car chases: it's still worth seeing, when it's done well, and Gaiman does it well.

I say things that happen to the protagonist because, as was pointed out to me, "the main character has no agency". This is true: he doesn't do things; things are done to him. I see this as making the situations scarier, since he's only seven and doesn't have a lot of control over his life under the best of circumstances. You can also see this as a story written from the point of view of the MacGuffin, or of the damsel in distress.