Reads like a contemporary From the Mixed-Up Files of Mrs Basil E Frankweiler (but not as good). I liked the layers in the book. I like that the characters are diverse though the introduction to Petra's MENA/WANA heritage is strange. I wish Balliet had spent a little more time developing that rather than handwaving Petra's race away.
My main complaint is that I couldn't really follow along with the clues in the illustrations since I read the book on my phone and the app I was in didn't allow for zooming in the illustrations. However, that's a format problem, not a story one.