By Jan Siegel. Alternatively titled Witch's Honor.
I don't think I knew that there was another book to the series when I read The Dragon Charmer.
Somehow this is a dark (but morbidly humorous at times) mix of modern and ancient fantasy. The ending may break your heart (I suspect it's the reason the average score on Amazon is so low), but as I just saw pointed out in another blog it makes sense as one of the only things Fern could do to reclaim her honor.
Fern is a desparately unhappy heroine in both the second and third books of this trilogy (I haven't read Prospero's Children); she does what she thinks she needs to, but she never really accepts being a witch. And she has very poor luck with boyfriends. On the other hand, it is possible to have a little sympathy for Morgus, because as evil as she is, she really is clueless and hopeless in the modern world... but only a little: she repeatedly ignores (reluctant!) warnings that Fern is a danger to her.
I don't know whether to recommend this series or not. Despite a bit of humor, the core of the story is dark with only "artificial light" at the end, foreshadowed by the telling and poetic "prayer" at the beginning of the book. Fern makes her own way without God and, I fear, that is a grave mistake. I highly doubt the author will relent in a sequel. I guess the best purpose of this book is to serve as a warning: this is what happens in a world and a life without a savior.
*** Update: SPOILERS. I will come straight out and say it: Fern kills herself, either figuratively or literally, by drinking the waters of Lethe (cf. the ending of The Orphans of Chaos and the storyteller in Earth Logic, and imagine Fern sitting around in limbo waiting for her body to die.) The ending implies but doesn't quite say that she dies physically as well, leaving an empty room. If not, she apparently loses 14 years of memories... maybe things will turn out differently the second time she grows up, or not. Her solution is not merciful or forgiving, but only meaningless oblivion. I am writing this to get it out of my head.