As a new public librarian, I keep encountering books (Some Desperate Glory by Emily Tesh or T. Kingfisher's Minor Mage) that read like YA. They have YA characters, are written at a YA level, are targeted to YA audiences.
And yet they are not YA books. In an almost similar vein, I think I grasp that the Romance genre has clear definitions that require more than just romantic or spicy content, but the taxonomic complexities of YA literature elude me. Can someone point me to resources?