Very sad news.
She was one of the first romantic suspense authors that I read when I was a kid. I think I found out about her because the Disney movie, "The Moon-Spinners" (with Haley Mills) was based upon her novel, and I loved that movie as a kid. I probably saw her name in the movie credits and looked her up in the library.
I think that I've read all of her books, with the exception of the Merlin books (not an Arthurian fan - except for Monty Python and the Holy Grail). "The Gabriel Hounds" was always my favorite.
Reading her books led me to Victoria Holt, Phyllis A. Whitney, Dorothy Eden, and Helen MacInnes. So thank you very much, Mary, and RIP.