This was an extremely fun read. It's so thoroughly imaginative, it's a great example of just what books can do: you can read a story about talking animals, castles, future societies, and just be in it, rather than distracted by CGI.
When I read Mr. Penumbra’s 24-Hour Bookstore, it reminded me of YA fiction like Cory Doctorow’s. This book feels more universal: I would still recommend it to my young second cousins, but there's nothing about it that feels markedly "young adult."
Overall: really great read. And a special note that I bought a hardcover (at my local exceptionally cute indie bookstore), and the physical copy is one of the most satisfying books I've owned in a while. The cover art is extremely rad, with embossed glow-in-the-dark lettering. Really knocks it out of the park on those details.