Obscure book opinion of the day: the asexual and aromantic communities are sleeping on the Mechanica duology by Betsy Cornwell (Mechanica and Venturess).
They're YA fantasy, published in 2015 and 2017 respectively, and the central relationship is a very explicitly queerplatonic throuple. We’re not talking subtext or headcanon here; it’s as clear as it can possibly be without using modern terminology.
It's less clear whether Nicolette, the MC, is intended to be ace and/or aro; I'm inclined to think she’s not. But it's emphasized over and over throughout both books that the bond between her, Fin (the handsome prince), and their mutual best friend Caro isn't romance in the usual sense. They're each other's most important people and fully expect to continue being so, regardless of any more traditional relationships they might develop individually with other people. Nicolette spends part of the first book thinking she's falling in love with Fin and being heartbroken when she sees that he and Caro clearly love each other, but she comes to realize that she was trying to fit her feelings into too narrow a box thanks to all the stories that claim romantic love is the most valuable kind.
The other aspects of the books are enjoyable too - a loose Cinderella plot with a heroine who rescues herself, steampunk inventions, Fey magic - but I don't think they would stand out to me nearly as much without the strong focus on a type of relationship that’s all too rare in fiction, especially YA fiction.
