Marked Series by Sylvia Day

Marked 01 – Eve of Darkness
Cursed by God, hunted by demons, desired by Cain and Abel... All in a day's work.
For Evangeline Hollis, a long-ago fling with a bad boy from the wrong side of the tracks just became a disaster of biblical proportions. One night with a leather-clad man of mystery has led to a divine punishment: the Mark of Cain. Thrust into a world where sinners are drafted to kill demons, Eve knows her learning curve must be short. A longtime agnostic, she begrudgingly maneuvers through a celestial bureaucracy where she is a valuable but ill-treated pawn. She's also become the latest point of contention in the oldest case of sibling rivalry in history. But she'll worry about all that later. Right now she's more concerned with learning to kill while staying alive. And saving the soul she'd never quite believed she had.

Marked 02 – Eve of Destruction
When you're learning to hunt demons, homework can be hell...
Class is in, but Evangeline Hollis is far from being the star pupil. She's struggling to get through the required training to be a full-fledged Mark -- one of thousands of sinners forced to hunt demons for God. When her class goes on a week-long field trip to an abandoned military base, passing the course suddenly isn't just a matter of pride... it's a matter of life and death. There's a demon hidden among them, killing off Eve's classmates one by one. As the body count rises, a ragtag team of ghost hunters from a cable TV program unwittingly stumbles into the carnage. Now keeping the Mark system secret competes with the need to keep the "paranormal researchers" alive. With Cain on assignment and Abel investigating a new, terrifying class of demon, Eve must fly solo on her hunt to stop a killer before he strikes again.

Marked 03 – Eve of Chaos
How do you tell Satan that you ran over his hellhound?
Evangeline Hollis has no idea and she doesn't want to find out. Living with the Mark of Cain—and the two sexy brothers who come with it—is trouble enough. She doesn't need to borrow more. Too bad Satan is too pissed to oblige her. Incensed at the loss of his pet, Satan has put a bounty on Eve's head, and Hell's denizens are converging en masse. The proliferation of Infernals is complicating Eve's hunts and creating chaos in her once orderly life. They've also brought her to the attention of an overzealous reverend who is certain she's Jezebel reincarnate. How can a Mark drafted by God strike a bargain with the Devil? Eve's about to find out...