In Search of a Hero by Terri Mackenzie

They have just one rule: don’t fall in love
Forced into an engagement to save her destitute family, Lady Theodosia (Theo) Beaumont gives up her dreams of romance and a husband with all his teeth. Thankfully, her friend Nathanial Hardinge proposes a solution: a marriage of convenience. He’s no romantic, but he’ll grant her freedom, an alarmingly handsome husband, and the title of duchess.
All she has to do is not fall for him.
To Nathanial, marrying Theo makes sense. They’ve agreed not to let emotions complicate matters; his life can continue the way it always has. There’s just one problem—now she’s in the picture, his old life no longer appeals.
When his cousin and heir attempts to woo Theo and destroy the marriage, Nathanial sets out to save her, and learns there is nothing convenient about marriage, love, or desire. And Theo finds that her husband might have more than just his teeth—he also has her heart.