As far as I know, Mary Stewart was a prolific writer.  Yet it seems as if I've already read most of them.  The following is a list of her books that I have read or currently have a copy of.  I would appreciate it if anyone knows of any other books and include it in the list:
- Wildfire at Midnight
- Madam, Will You Talk?
- Thunder on the Right
- Nine Coaches Waiting
- The Gabriel Hounds
- Airs Above the Ground
- This Rough Magic
- The Moon-Spinners
- My Brother Michael
- The Ivy Tree
- Touch Not the Cat
- A Walk in Wolf Wood
- Thornyhold
- Rose Cottage
- The Stormy Petrel
She also wrote the Merlin books:
The Crystal Cave
The Hollow Hills
The Last Enchantment
The Wicked Day
I am unfamiliar with her juvenile fiction books.  Have I missed anything?  I would love to find any other books she may have written.
Yes, you're missing a hidden Gothic gem!
She wrote a small book that is essentially a novella (about 100 pages or so) called "The Wind Off the Small Isles." It's almost like an early draft from one of her other classic Gothic novels, but, for whatever reason, she didn't expand it to full-length.
It was printed and released in the UK by Hodder & Stoughton and I bought my copy on ebay a few years ago. Copies can be hard to find (it was only ever released in hardcover) but they are out there. It's worth it too if you can't get enough Stewart. It's not her best work, by any means, but it's as good or better than her last couple of so-so novels.
I also recommend her juvenile novel "A Walk In Wolf Wood." Not a Gothic, exactly, but it's perhaps her one book that's a combination of the King Arthur and the romantic suspense stuff. It's an enjoyable read if you can't get enough Stewart.
She also wrote two other juvenile novels: "The Little Broomstick" and "Ludo and the Star Horse," both worthy reads but much more juvenile in tone than "Wolf Wood."
I think Airs Above the Ground is my favorite. I love lipizzaner horses. The Gabriel Hounds is good too, loved the ending of that one.
I've read five this year, The Ivy Tree, and Nine Coaches Waiting, both of which were traditional Gothics which I enjoyed very much. I also read Airs Above the Ground which was good, but moving away from Gothic to Romantic Suspense. The climatic scene was a chase through an old Austrian castle. I also read This Rough Magic and My Brother Michael both of which are not Gothic but Romantic Suspense, both books bogged down in long dialogue about war and smuggling and bored me too tears. I do not recommend these last two books. I will say that she is one of the best descriptive writers ever, I totally had a sense of Austrian mountains and valleys, north Yorkshire, or Grecian hills along the Mediterranean.
I just found The Ivy Tree today at a library book sale. I have never read it, but am glad to see that it is on the list of books which you enjoyed!