"
Quest for the Well of Souls" has someone who becomes a fairy-like creature, although it's also a TG transformation, if I recall correctly (and I'm not sure you'd describe a "Well of Souls" book the way you did -- there are a myriad of transformations in most of them, and they're almost all "poof" transformations). One of the editions of that book did use her as the cover subject:
(Art by fantasy illustrator Clyde Caldwell)