The Gilded Cage by Edith Layton

Post-Civil War New York was a golden city where wealth waited for the bold and everything had its price. It was here that well-born but impoverished Lucy Markham sought fame and fortune on the stage under a false name -- and became the toast of the town and the target of every man-about-town. It was here that handsome, iron-willed millionaire Josh Dylan came to put the seal on his success by conquering society. The moment Josh saw Lucy, he wanted her -- but only as his mistress. The moment Lucy saw Josh, she fought to quell her desire for him. For neither could afford to surrender to the one thing that threatened their carefully laid plains. Love.