Alphabet of Thorn, Patricia A. McKillip
Firstly, I think I’ve mentioned this before, but oh I love the cover art so much. It’s done by Kinuko Craft, who has also illustrated at least some of Juliet Marillier’s covers, so that explains why it seemed familiar.
Alphabet of Thorn is beautifully written. It’s one of those books where it’s less about making things happen, and more about watching them happen — there is some degree of “stopping things happening”, but mostly people fall in love, and do magic, and learn things about themselves, and work in the slow silent world of manuscripts and translation, and… It’s not really about the plot, I think, but more about the characters and the world; less about things happening than about people in a situation. I don’t know if I can describe it — but all the same, I hope it’s clear that I loved it.
The mythical parts of the story, the floating school of magic, all of it felt — not real, but true, to me. It all worked seamlessly as a world, as a story. It also felt in some ways like something Ursula Le Guin could write (which is a very high compliment in my world): the Floating School is a cousin to the school on Roke.
If you’re not a fan of McKillip’s style and plotting, I don’t think this one could change your mind. But I do think it’s gorgeous.
Bibliophibian Inc. 
Another recommendation that I need to follow up. Thanks!
I hope you like it!