All You Can Handle (The Solars Duology, Book 2). The Solars Duology, #2

Alec Ross lived in his sister's shadow until he was recruited to join an underground organization dedicated to keeping peace between solars and humans. Pax Solis gave him purpose in his work as a handler and a way to keep his deepest, darkest secret from ever being uncovered. If his feelings for his top agent are getting complicated, well, no one ever has to know that, either. Delta Verrett has always known her path.
The child of two high-ranking Pax Solis agents, she grew up in the secret organization and now works to contain any threat by those with extraordinary powers. Alec is the best handler she's ever had, and their track record is impeccable. If her attraction to him is getting inconvenient, well, maybe it's time to address it head on. But "addressing" it triggers something dangerous, and the timing couldn't be worse.
Pax Solis is facing its biggest challenge in Helios, a group determined to steal solars' power and who may have compromised Pax. And that something dangerous may mean Alec and Delta can never be together.just when they're coming to realize they don't want to live without each other.