Seduced by Moonlight by Laurell K. Hamilton

To some I am Meredith Gentry, P.I. To others I am Princess Merry, heir to the throne of Faerie. And there are those who whisper that I am both of these and more.
My aunt, the Queen of Air and Darkness, no long distracted by her sadistic pastimes, now focusses unwaveringly on me. I spend each night with my immortal guards, but still no child has come of our decadent pleasures. But something IS happening to me. I appear to have awakened a force that's lain dormant for thousands of years and I haven't the damndest idea how or why...
It all began with the chalice. I dreamed it, and there it was - cool and hard - when I awoke. My guards know this ancient relic well - its disappearance so many ages ago stripped them of their vital powers. And here it is with us now. My touch resonates with its force. A strange, dazzling magic now courses through my half-mortal half-Sidhe body. But while my guards cherish me for this unexpected gift, there are those who loathe me for it and would rather the Unseelie court suffer than have it ruled over by me, a mongrel queen. My enemies grow in number every day. If only they knew what I am capable of. But come to that, if only I did too.
This is the world of Meredith Gentry, a twilight world of gods, shapeshifters and immortal souls, a world full of sensuality, wild magic, treacherous deceits and latent powers about to be unleashed...

This book is part of the "Merry Gentry" series.
Here are some other books from this series: