1) Dick Francis
Francis passed away not too long ago but his books are still great reads. There are tons of them set in the British racing world, mysteries, but very well written. You can find these often in antique malls for a few dollars, or online.
Fellow Virginia Rita Mae Brown is an avid fox hunter (she has her own pack of hounds) and she writes mysteries set in the hunting world.
3) Tami Hoag
Hoag is also a mystery writer.
4) Jane Smiley
Smiley is a rider and horse lover and has several books about horses.
5) Other Lesser Known Writers
Here are some books that I am aware of, some I have read, others have been recommended.
Nancy Shulins - Falling for Eli
Josh Pons - Country Life Diary - this is one of my all time favorite books, written in a three-year diary in narrative form, about life on a breeding farm in Maryland. Pons writes very well and I did not want to put this one down. I believe it is out of print but it can be found on Amazon or on used book sites.
Barbara Moss - In and Out
Kathleen Brooks - Bluegrass State of Mind