Devil in Spurs by Norah Hess

One stormy night Cord discovered that Jonty would never be a man, only the wildest, most willing woman he'd ever taken in his arms, the one woman who could claim his heart...

Raised in a bawdy house, lovely young Jonty Rand had posed as a boy all her life to escape the notice of the rowdy cowboys who frequented the place. And to Jonty's way of thinking, the most notorious womanizer of the bunch was Cord McBain. So when her granny's dying wish made Cord Jonty's guardian, she despaired of ever revealing her true identity. Worst of all, Cord was determined to change her from a "sissy" into a rawhide-tough wrangler. In the rugged solitude of the Wyoming wilderness he rode his young charge relentlessly, assigning Jonty all the hardest tasks on his horse ranch and making her life a torment.