Shades of Buffy’s friend Anya, perhaps? That’s not really fair, because this story is original and fun.
Succubus Blues by Richelle Mead is a fun read with a touch less steaminess than you would expect from a book about a sex demon.
Our heroine (sadly no I don’t remember her name) is a succubus who really seems to prefer her day job in a book store.
She has a major crush on an author, but can’t let herself fall for him because she can’t help draining the life energy out of anyone she gets physical with.
There’s a nice pantheon/organization of the angels and demons, but the baton of exposition is lightly wielded.
I’ve tagged this as mystery as well as fantasy, because there’s a who-dunnit of murdered demons and angels. Our heroine is perhaps a suspect, but we know she didn’t do it.
There were some hints dropped about atonement and demons possibly being able to rise above evil – perhaps our protagonist is on the way to doing something special, because she really does not seem to be the evil type.