Splendors and Glooms by Laura Amy Schlitz
https://www.goodreads.com/en/book/show/13531021-splendors-and-glooms
This one completely captivated me. I've read other books by her that I've been less enamored by, but this one is great.
I started reading middle grade fiction when my kids were in their early grades, to build a library of books they might enjoy ahead of time. But I think I liked this book better than either of my kids did.