I'm always interested in finding authors whose books are similar in some way to Mary Stewart's. Same sort of tone, similar plots or settings.
Here are some authors that I've tried and can recommend as very good:
Barbara Michaels is a pseudonym of Barbara Mertz, who also writes historical fiction under the name Elizabeth Peters. Under the Michaels name she writes gothic suspense novels, which often have romantic threads running through them.
Daphne du Maurier is most famous for Rebecca, which is an amazing book. I haven't read any of her others but mean to in the future.
M.M. Kaye wrote a series of "Death in" books. Death in Kashmir, Death in the Andamans, etc. All feature young English women in exotic settings caught in dangerous situations. (She also wrote my favorite children's book of all time, The Ordinary Princess, which is not at all like anything Mary Stewart ever wrote but still fabulous.)
Susanna Kearsley writes lovely suspense novels, some contemporary and some historical (and some a combination of both!). My favorites are The Shadowy Horses and The Winter Sea.
And a few other authors I've seen mentioned as similar to Stewart but haven't read:
Victoria Holt, Dorothy Eden, Phyllis Whitney
Anyone have any other suggestions?