Hell On Earth, book 3: Hotter Than Hell
Jackie Morse Kessler2008

So whose soul do you have to damn to get a promotion around here?

Daunuan was never the ambitious type. There's so much to love about his job just the way it is--mind-blowing sexual prowess, the power to seduce any human, excellent dental plan. But now Pan, the King of Lust, has offered to make Daun his right-hand incubus--a position other demons would give their left horn for. All he has to do is entice a soul destined for heaven into a damnable act of lust. Should take, oh, seven minutes, tops.

Then he meets his target, Virginia Reed. She's cute. Funny. Smart. Unfathomably resistant to his charms. He can't understand it. But Daun has centuries of seduction to his credit. He's the best there is. Sooner or later he'll transform this polar icecap of a female into a pool of molten desire, and every instinct tells him she'll be worth the effort.

Meanwhile, he has to deal with a plague of rogue demons Hell-bent on taking him down, sent by an unknown enemy with a serious grudge. And one other problem: the dawning realization that he's falling in love--that unholiest of four-letter words--with the woman he's about to doom for all eternity...