Dark Ones, book 04: Even Vampires Get the Blues
Katie MacAlister2010

Paen Scott is a Dark One: a vampire without a soul. And his mother is about to lose hers too if Paen can’t repay a debt to a demon by finding a relic known as the Jilin God in five days.
Half-elf Samantha Cosse may have gotten kicked out of the Order of Diviners, but she’s still good at finding things, which is why she just opened her own private investigation agency.

Paen is one of Sam’s first clients and the only one to set her elf senses tingling, which makes it pretty much impossible to keep their relationship on a professional level. Sam is convinced that she is Paen’s Beloved—the woman who can give him back his soul...whether he wants it or not.