Oh, YEAH! One of our daycares had it. It is told from the horse's point of view -- he is a retired racehorse (where he also went in a circle). Very touching, does not quite explain hippotherapy to a kid though.
Sorry, I haven't read the book. But here's a social story with simple illustrations about riding horses at a camp for kids with disabilities such as autism.
http://www.mhkc.org/media/doc1/Autism%20Social%20Stories/RID ING%20HORSES%20AT%20CAMP.PDF
Good luck with everything.
We Go In A Circle - Peggy Perry Anderson
It is the only book I can find on hippotherapy. I am not entirely comfortable with it, but I can't find anything else. I wondered if the book was better in reality than the few glimpses they give of it on Amazon, because something sort of squigs me out about the pages I am seeing, but it might be fine on the whole.
I am trying to find a book about horses for my son, who might be starting hippotherapy soon, but they all seem to be geared towards girls who are going to do jumping and things like that.
Any other suggestions would be helpful too.