Love Slave by Bertrice Small

It is no act of love that Regan experiences the night she takes her sister’s place in the marriage bed, a counterfeit bride to sate the lust of her twin’s new husband. Come morning, her sister, carrying another man’s child, resumes her place and Regan is spirited away to a nunnery.
But a more exotic fate awaits the fiery Celtic beauty: Regan is sold to a slave trader. It will be her destiny to come under the tutelage of Karim al Malina, master of the erotic arts, who will mold Regan — now renamed Zaynab, the beautiful one — into a Love Slave fit for a Moorish king, though the pair break the first rule of teacher and pupil . . . they fall in love.
But Zaynab is not Karim’s to keep. She is given to the Caliph of Cordoba, who vows to love her . . . and pleasure her as no younger man can. Yet Regan still longs for her one true love, Karim al Malina, and vows that, somehow, their fates must be reunited . . . .