This was Doranna Durgin's first published novel, and one of her best. She has subsequently published about ten novels, two of which are direct sequels to this one. The three books stand on their own, but can also be read as a trilogy, which is
Dun Lady's Jess
The Changespell
Changespell Legacy.
A park ranger and his partner find a confused young woman in the woods, naked except for a horse's saddle and harness, and unable to speak. At first they think she is hurt or deranged. This opinion is reinforced when, as they try to communicate with her, she picks up a few words of English and tells them that she is a horse. Little do they know that she has been sent to our world, and transformed into a woman, by a spell gone wrong, and from a magic world where she really is a horse called Dun Lady's Jess. Her struggle to make sense of her new environment is a fascinating story.
Doranna Durgin knows more than a little about horses, and it shows. I found this to be a charming and very enjoyable book, and would particularly recommend it to anyone who loves both fantasy and horses.