Beneath The Sugar Sky by Seanan McGuire (Wayward Children, #3)
Beneath the Sugar Sky attempts to fix something broken before it loses all possibility of ever being right again. There's a sense of urgency defined by absence rather than presence; tension created by what ought to be but is not.
The world-building is fantastic, both for the specific places they visit and for what those places say about the multiverse as a whole. It moves along with the best answers we have for now and then tells the reader new answers as the characters discover them. It made it feel like discovering something wonderful along with people who know enough to be guides while still having a sense of awe at every new thing.
This returns to the setting of the first book to advance what looks like it will be the main narrative of the series, tracing the lives of the children at the school during their tenure (while the even numbered books tell more about people during the time they did not reside there). I knew a little of that structure but I was pleasantly surprised by the variety of settings in this one. The school is a locus but it doesn't constrain the story at all, getting out of the way so that we could have a quest this time. I like seeing different sides of old characters and meeting some brand new ones.
CW for (internalized) fatphobia and references to dieting.