Fireflies by Melissa Koberlein

Ghosts don’t exist but aliens do, and they’re living inside my boyfriend. Will has extraordinary abilities thanks to the beings living inside his body. His job is to recruit rare compatible humans to join his kind. His latest search leads him to the spirited Marley. The more time he spends with her, the more certain he is that she’s meant to be his. Meanwhile, she can’t stop dreaming about him. An alluring glimmer in his eyes reminds her of fireflies. When she discovers his true nature, her future is altered forever.