I'm not sure if any of these are what you are looking for, but I love to read anything I can get my hands on about horses and these are some of my favorites.
She Flies Without Wings by Mary D. Midkiff
A Bridge Called Hope and Hope Rising both by Kim Meeder
Chosen By A Horse by Susan Richards
Chosen Forever also by Susan Richards ( I haven't read this one yet but I loved the first one so much)
Sometimes A Woman Needs A Horse by Betsy Talcott Kelleher
Why We Ride: Women Writers on the Horses in Their Lives -Verna Dreisbach