Bitten - Kelley Armstrong

Elena Michaels is a model woman for the 21st century: self-assured, keenly intelligent, fighting fit. And like every modern woman, she has her secrets. Nothing extraordinary about that. Except that Elena really is extraordinary. In fact, she may well be the most extraordinary woman alive. She is, after all, the only female werewolf in the world...
Ten years ago, against her will, Elena's lover turned her into a werewolf. Some days it feels like a gift. Most days it feels like a curse. A year ago, she decided to live as a human. Now she has to go back to New York State, her old home. Her pack is under seige by a new group of violent, psychotic werewolves that shows no respect for the old ways, and no respect for territory. Forced into helping her old friends, Elena soon slips back into the reassuring camaradarie of the pack, though she struggles against her dangerous, unpredictable desires.
Hunting down her enemies, Elena prowls through territories usually barred to women. From dangerous back alleys to the dark, luscious forests of New York State, she must hunt and destroy the renegade pack before they destroy her.