The Viscount Can Wait by Marie Tremayne

After five years away, Lady Eliza Cartwick isn’t relishing returning to the whirl of the London season. But the young widow knows to ensure the best future for herself and her young daughter, Rosa, she must remarry. If only Lord Evanston, the dashing rogue who has haunted her dreams since she was sixteen, didn’t insist on distracting her with his searing looks and lingering touches at the most inconvenient times . . .
Thomas, Lord Evanston, has wanted Eliza since her engagement ball all those years ago. His best friend’s sister has constantly been out of reach . . . until now. The forbidden has always tempted him, but when Thomas realizes he wants the object of his fantasies for far more than a dalliance, he must convince her that he’s not just a rake; he’s a viscount who’s worth the wait.