Nightshine, by Lynn Viehl; copyright 2011 by Sheila Kelly; ISBN 978-0-451-41314-7.
Part of the Kindred series, which also includes Shadowlight, Dreamveil, and Frostfire.
I gather the Darkyn series is related. It includes at least If Angels Burn, Private Demons, Dark Need, Night Lost, Evermore, Twilight Full, and Stay the Night.
Random musings on books, code, and tabletop games.