Witchling by Yasmine Galenorn
Everything I'd read about this one said it was really good. I agree, this was really a good book. The first in a series about three sisters, half human, half faery, all with different paranormal talents. The sisters are operatives for the Otherworld Intelligence Agency. This book focused on Camille, the witch. Because of her mixed blood, sometimes her spells don't work quite right.
I'm looking forward to reading the next book in the series, Changling.