Cimorene

Behold, the book list (http://www.leemac.freeserve.co.uk/books.htm) on the worst author's website ever! (Why do writers always have such terrible websites? Is it a requirement?) I went there to confirm the title of Sudden Wild Magic; I'd thought she had only written two adult novels, that and Deep Secret. But then I found out about Changeover and that she considers Dark Lord of Derkholm adult; I'd thought it was YA.

Deep Secret is, I think, considered adult because a) the narrators are past YA-hood themselves and b) it's longer than her usual YA books, though the matter is for me made somewhat more confusing by its having sequels that are YA. A Sudden Wild Magic I'm not at all sure about - age again, maybe? And Changeover could be considered adult for any reason, including graphic sex scenes and loads of lovingly-detailed torture sequences (although this really wouldn't separate it from a lot of YA books currently being published, actually), since its existence cannot be independently verified. (Seriously. Years of searching everywhere for her books, in England and the US, and I'd not heard of it until I checked her website.)