Pembroke Park by Michelle Martin

When Lady Joanna Sinclair meets Lady Diana March on horseback and clad in male attire, she is outraged. Such bizarre behavior is simply unacceptable in Herefordshire:

But she is irresistibly drawn to the headstrong Diana, whose eccentricity cloaks a mysterious darkness in her past. And Joanna learns that Diann's coterie of 'unusual' friends has among them her own brother-in-law, who is in head long pursuit of the beautiful and elusive Geoffrey. Under Diana's influence, falling ever more deeply in love with her, Joanna asserts her independence from her brother, the arrogant and overbearing Hugo, who vows to subdue both of these defiant women. But Hugo is up against more than he bargains for in Lady Diana March...

Engaging and erotic, this colorful novel portrays England's regency period when, regardless of wealth or title, to love one's own sex was a risk taken only by the most daring...