Lover, Divine by A. Star is the first book in her Mythos: Gods & Lovers series. Apparently this is a spin-off or connected series to her young adult series The Oracle of Delphi written as Diantha Jones. 
 This book is most definitely ADULT. 
 Siobhan's family acts as agents on earth for the Greek gods. When immortal Liam shows up and starts seducing Siobhan she has a hard time resisting. She later learns that he is really the Greek God Apollo. 
 With many twists to the plot and plenty of god-driven subterfuge, the story is a steamy tale of unconventional relationship between two destined lovers and the repercussions of divine love. This paves the way for the next book in the series. It's intense and the mythology, while about Greek gods, is it's own version of that Greek lore.