Lament — Maggie Stiefvater

Sixteen-year-old Deirdre Monaghan is a prodigiously gifted musician. She's about to find out she's also a cloverhand -- one who can see faeries.

Unexpectedly, Deirdre finds herself infatuated with a mysterious boy named Luke who enters her ordinary life, seemingly out of thin air. But his interest in her might be something darker than summer romance. When a sinister faerie named Aodhan shows up with deadly orders from the Faerie Queen, it forces Dee right into the midst of Faerie.
Caught in the crossfire with Deirdre is James, her wisecracking but loyal best friend.

Deirdre had been wishing her summer weren't so dull, but taking on a centuries-old Faerie Queen isn't exactly what she had in mind.