Demon's Song (Hearts of the Fallen) by Kendra Leigh Castle:
The road to Hell is paved with good intentions. At least, that's how it seems to Sofia Rivera when her attempt to help a friend ends in a blood-soaked confrontation with creatures that aren't even supposed to exist. With a group of rogue vampires determined not to leave a single witness to their violence hot on her heels, Sofia finds herself saddled with an immortal bodyguard—Phenex, the fallen Angel of Song.
While the vampire king roots out the corruption threatening to take down his underworld kingdom, Sofia will have to deal with a fallen angel whose dark beauty is often at odds with his ornery nature. Slowly, Sofia and Phenex give in to the strange attraction neither can deny. And as the darkness closes in around them, Phenex begins to wonder whether even the wickedest Fallen might find his own happyending...and his heart.