A Caress of Twilight by Laurell K Hamilton (2002) Ballentine Books

Meredith Gentry is an L.A. private eye.  She’s also a fae princess.  To become queen, she must bear a child before her cousin can father one of his own.  So she proceeds to attempt the feat with the warriors of her royal guard.  And she has to stop an ancient evil that is trying to destroy the fabric of reality.