The Fever books are a series written by Karen Marie Moning.
The series revolves around MacKayla Lane, a seemingly normal girl of 20-something living in small-town Georgia, USA. When her sister is brutally murdered while studying abroad in Ireland, MacKayla resolves to track down the killer by travelling to Dublin. When she arrives, however, she quickly realises that all is not what it seems in the Irish capital, nor is she all that she thought she was. For one, she learns that the Celtic folklore concerning 'fairies' is true, but not in a pleasant way: not only are the real Fae a lot more powerful and dangerous than what the bedtime stories make them out to be, but millennia ago, they split into warring factions of Light (Seelie) and Dark (Unseelie), and that war is now threatening to spill over into the human world. For another, MacKayla discoveres that she is a sidhe-seer (a person who can see the Fae for who they are, even when they are casting a 'glamour' in order to appear human), and that she is expected to take up the fight against the Fae in order to preserve the human world.
As she delves deeper into her search, she realises that her sister's death is linked to the search for dark and mysterious book - the Sinsar Dubh - which has gone missing from the Faerie realm. Believed to hold all the magical knowledge of the Unseelie King (including the power to create life) many others are attempting to track down the book as well: the wealthy and enigmatic Jericho Barrons, the tantalizing Seelie Prince V'lane, the powerful 'Lord Master' Darroc, the secretive Sidhe-seer society, and others. In order to avoid the fate of her sister, and to save the world from impending disaster, MacKayla must not only distinguish friend from foe in a field where no one is what they seem everyone is out for their own gain, but also to come to terms with her new-found identity as a sidhe-seer.