The Duke’s Counterfeit Wife by Caroline Lee

How did she end up make-believe married to the Grump Next Door?
Spinster scientist Felicity Montrose has always been as curious as one of her pet cats. So when her dark and dangerous neighbor catches her breaking into his house, her body’s reactions to his manhandling raises all sorts of interesting questions. Unfortunately, the irritating man refuses to help her in her experiments to replicate these titillating sensations. Blast.

Griffin Calderbank is well aware that his enticing next-door neighbor is a lady, and thus so far off-limits she might as well be in Scotland. A man like him, a man who spent years fighting for his life and surviving with his fists, needs to focus on keeping his family safe. She’s a dangerous distraction he can’t afford, no matter how tantalizing her offer might be.
His hands-off policy is working well, right up until their children enter them into a contest to give him a chance at the inheritance long denied him. The only catch? He’ll have to pretend to be a happy husband to the odd, but oh-so-tempting Miss Montrose. And this time, when she makes him a scandalous bargain, there’s no point in refusing. After all, she is his wife, right? For a short time at least.

Soon, however, it’s not just a dukedom on the line, but the very future of the country, as a hunt for a traitor turns deadly. How can he be expected to concentrate on such stakes when there’s a brilliant, quirky scientist in his bed, offering him her cooperation in exchange for satisfying her…curiosity?