By Holly Black.
This book is not worth reading, at least in itself. Unfortunately, it isn't always obvious: the main character is sympathetic because she is young and bewildered and tossed about by compulsions (literally, enchantments) that she has no control over. At the same time, though, she seems incredibly childish; she repeatedly gets herself into trouble she could have avoided had she been wiser. And, really, she should know better.
I would also criticize the frequent crass language and profanity, but I can't argue if you say it fits the characters perfectly: it is certainly in keeping with the kind of teenagers they appear to be. However, it doesn't stop me from wishing they were better. As it is, the main character could easily be one of the extras who dies; she seems little deserving of salvation aside from the fact that she is the main character. Her main virtue is either courage or rash foolhardiness combined with impulsiveness. She also smokes, until her physiology forces her to stop...
So why do I say "in itself"? Well, I have hopes that maybe Kaye will grow up in the next book more than she did in this one. (Yeah, so when has that ever happened?) We'll see.
A side note about the subtitle: the book is loosely based off fairy lore, especially Tam Lin (hence the title), but the majority of the plot is not strongly patterned after any specific tale (at least that I recognize).