OMW, I love Love LOVE Avalon. I first read it when I was just a kid, in one of those Readers' Digest condensed books, complete with illustrations. It haunted me and still does. Amazing, isn't it, how the best books stay with you for years, if not decades? Seton wrote like that.
Her other titles: Green Darkness, a reincarnation based paranormal. Good, but not on par with the others. It does NOT have a HEA. The Winthrop Woman, about the early settlers of Massachusetts. The Turquoise, which for some reason I've never read. She was a terrific writer who knew how to use simple, spare prose in an amazingly evocative way.