I thought the series was by Tannith Lee, but looking over her books, I can't find it. I believe it was a trilogy, and the copy my mom had was from the SF&F Bookclub. It starts out with a girl being sacrificed or captured by some kind of gargoyle, and taken to a place in the middle of the desert. I believe there were several other women already there, and it was a Beauty and the Beast kind of setup at first. The girl ends up freeing the gargoyle of his curse, IIRC, and later goes on to become a very powerful witch of some kind.
It's been about twenty years since I read the series, so I'm a bit light on the specific details. Any ideas?