Fallen Angel: The Thorn Chronicles, Book 3

Sixteen-year-old Sunday is a powerful Shade who is itching to use her power. But her mentor, Naomi, won't let her go on jobs with the Guardians, so she spends most of her time kissing boys behind the greenhouses, showing up late to lessons, and swimming with her cat. When Naomi asks Sunday to do a job for her, Sunday jumps at the chance, only to find out it's in Tombstone, Arizona - where adventure sounds as likely as the town's name. Once there, Sunday pokes around and she finds an organization that threatens not only her, but all female Shades. Determined to prove herself to Naomi, Sunday goes after the group on her own, only to discover that Tombstone might live up to its name after all.