For a long time, Mary Stewart titles were mostly non-existent and/or very expensive in the US, but now there seem to be a lot of them available for reasonable prices. Right now, there are three titles (including my most favorite) that have dropped to $0.99 at Kindle and Kobo US, and a bunch of others in the $1.99 to $2.99 range.
Mary Stewart was one of my earliest favorite authors, so I got a bunch of her e-books one time while on a business trip to the UK, where they've been much more available for a long time. I'm never quite sure whether to call her books romantic suspense (and post in mysteries/thrillers), or to call them suspenseful romance (and post them here), but in the end, this time, I went with suspenseful romance

, and will cross post to mysteries.
Madam, Will You Talk (my favorite Mary Stewart title, although I have a few other favorites that come close...)
Kindle US:
https://www.amazon.com/dp/B00GW4P044
Kindle US/Smile:
https://smile.amazon.com/dp/B00GW4P044
Kobo US:
https://www.kobo.com/us/en/ebook/madam-will-you-talk-1
Touch Not the Cat
Kindle US:
https://www.amazon.com/dp/B00GVFUEOU
Kindle US/Smile:
https://smile.amazon.com/dp/B00GVFUEOU
Kobo US:
https://www.kobo.com/us/en/ebook/touch-not-the-cat-1
Nine Coaches Waiting
Kindle US:
https://www.amazon.com/dp/B00GW4OZH2
Kindle US/Smile:
https://smile.amazon.com/dp/B00GW4OZH2
Kobo US:
https://www.kobo.com/us/en/ebook/nine-coaches-waiting-1
I'll do links for the $1.99 and $2.99 titles in a bit and put them in another message.