Stonehurst by Barbara Johnson

Come to Regency, England and revel in the year's most colorful, sexy and entertaining love story. Alex, her dark chestnut hair cropped short, her tan breeches showing off her calves and thighs, teaches Lady Cassandra more than riding!
----
Lady Cassandra lives on the family manor in England, busy with social obligations and life of the wealthy. Alex works at the manor's stables, making her way through life as best she can. When Alex helps Lady Cassandra onto her horse they both feel a rush of feelings. When Cassandra's visits to the stables become too frequent, her daddy the Lord of the manor arranges for her marriage to Lord Alfred. But - don't underestimate the determination and courage of two young women in love...