Third Husband's the Charm (Harrow's Finest Five #3)
Nancy Yeager

Fun-loving Percy, the Captain Lord Granston, is everyone’s favorite cad. But his easy charm hides a dark secret. He’s plagued by nightmares and fears he’s losing his grip on reality. Then his wife arrives in London. The only problem is, he doesn’t remember marrying the woman.
Twice-widowed Finola Tenney Simmons has lost her hard-earned fortune. But she’s devised a scheme to reclaim it from her greedy stepson. All she needs is a pretend husband, with her old acquaintance Captain Granston playing the part. There’s just one complication. The captain inexplicably believes they’re actually married.
When Percy learns Finola isn’t his wife, he agrees to keep up the lie to secure her fortune, if she helps him settle his family affairs before he loses his mental faculties. But living as husband and wife leads them into temptation, and they devise a new plan. They’ll keep each other’s secrets, embark on a brief affair, and both get what they want before their fake marriage ends. It will work like a charm, as long as neither of them falls in love.