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:

1. Wildfire at Midnight
2. Madam, Will You Talk?
3. Thunder on the Right
4. Nine Coaches Waiting
5. The Gabriel Hounds
6. Airs Above the Ground
7. This Rough Magic
8. The Moon-Spinners
9. My Brother Michael
10. The Ivy Tree
11. Touch Not the Cat
12. A Walk in Wolf Wood
13. Thornyhold
14. Rose Cottage
15. 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."