I wasn't sure what to expect from a YA Jeff Vandermeer book, but essentially this is over six-hundred pages of Jeff Vandermeer. If you like Jeff Vandermeer's other books, you'll probably love this. If you don't know who Jeff Vandermeer is, you should know that unlike some writers of epic young adult fantasy, Jeff Vandermeer is an excellent prose stylist. He's also incredibly imaginative, and leans into the weird pretty hard.
This book, like all his books, has its own style. It's told in a sort of British-clever voice, and is at once that, and a send-up of that. There are also sections of this book that are just beautifully written, and caused me to go back over them just to see how they were put together to create the effect they have. There are plenty of likeable and strange characters, and the world being set up is full of unexpected surprises.
Although this book in some sense has the spine of a typical fantasy quest, the details and rules of the world being built here really distinguish it from other fantasy. This book features talking marmots, Alistair Crowley, the disembodied head of Napoleon, and a slew of other characters I don't want to spoil. As I understand it, A Peculiar Peril is either the first part of a duology, or the first two parts of a tetrology. In any case, the book comes to a satisfying close, but there are plenty of interesting plot threads that need resolving.