Before I even arrived on the scene, the staff of vet library had arranged all of the books by subject. So far I've cataloged through over 600 books about:
Horse tack, including several volumes on bits, which as a horse trainer I find very useful (although I haven't read any of them, so I talk big....).
Barns, building them, picture books of lovely barns.
Carriage driving. Lots and lots of books on the instruction of carriage driving, where you could drive a carriage, etc. (Many of these can be seen in the Internet Archive)
I am now cataloging through the memoirs section. There are quite a few books on the royal family, but I am now directly in the middle of the "Women" memoirs. I am utilizing the Library of Congress classification scheme. Although Women in Memoirs had been sorted through, LC doesn't have a category specifically for women in equestrian sports, instead they are spread throughout the collection according to the topic they are talking about (eg. hunting, racing, dressage). This is, I find, quite fitting, as within Equestrian Sports, there is no distinction between men and women. In the Olympics, men and women compete with each other.
On that note, it is time for me to go off to the barn to ride my own horse on this gorgeous day!