Secret Mates by Juniper Hart

Can a human be trusted with a secret that’s been kept for thousands of years?

Reef Parker’s recent life has always been focussed strictly on running the Hollows Authority, the privately owned and only sector of law enforcement in Hollow Earth.
His life takes an unexpected and shocking turn when he learns an industrious sprite has developed a way to do the unthinkable—transport mortals into the Hollows.
Penelope Carling has lived her life without making waves. Shyly beautiful, no one could have imagined that beneath her quiet exterior was a tormented soul who had endured abuse at the hands of a gaslighting boyfriend for years.
Finally gathering the courage to end her relationship, her ex systematically ruins her life and sends Penny running back home to her roots…and into the most terrifying place she’s ever been.

Guided by an obscure dream, Reef knows he needs to keep Penny safe, but how can he when the whole world is out to get her?