I pick up every book I come across that has a horse, or horse-related item on the cover. 
Too bad you can’t search Amazon that way. However, you can follow the “also bought” links and check the listmania! links and find quite a few titles that way. Even if they’re not in print, you can at least find out about the book and most now have used copies for sale.
Actually, this BBS has been a great source of new titles. I’ve found lots of great books here!
Also, a friend of mine deals used horse books on the side. My ‘pusher’ always emails me when she’s got a title she thinks I’ll like. 