I was going to suggest selling them yourself on Amazon (find the book and look over at the right for the "Have one to sell? Sell on Amazon" button) but come to think of it, you can find a list of retailers that way -- just look for who is already selling books similar to yours.
Join paperbackswap.com and trade them free for other books on any subject under the sun. Judging from the number of wishers on horsey items on my wish list, horse books are in demand. I just got a horse book wish granted last week. All sorts of books, horsey and not, are in trade there daily.
sell them on ebay!! there aren't many horse books available on ebay, and I think if there were, they would sell well!! especially if they were below market value - i.e. if amazon sold a book for $20, list it for $10 on ebay and see what happens!!
I have a bunch of horse books I want to sell. I looked on ebay but the prices are pretty low. The others I'm going to look into selling with them. I'm tired of people telling me they will buy them but I never see anything in my paypal account!! HA!!