Lacking Natural Simplicity

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.