An Improper Encounter by Erica Taylor

Lady Sarah Hartford is beautiful, wealthy, widowed, and bored. After agreeing to a scandalous dare to seduce a stranger into her bedchamber, she meets the devilishly handsome doctor, William Gordon. Though both keep their true identities hidden, an intense romance blooms between them, and Sarah finally feels like she could be happy again.
Instead, William disappears from her life only to arrive three months later on her doorstep—married, a duke, and with a pregnant duchess in tow. The pair must try to untangle truth from lies while keeping their history a secret from everyone around them.