The Truth About Dukes by Grace Burrowes
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Robert Rothmere is hiding a past no duke should have endured, but he's not hiding it well enough. Sooner or later, his enemies will learn that he spent years locked away at a private asylum. To get their hands on his wealth, they'll try to send him right back to his worst nightmares.
If Robert is to foil their schemes, he needs to marry a perfectly proper, blessedly boring, deadly dull duchess, immediately - and he knows exactly which quietly delightful lady he'd love in that role. Lady Constance Wentworth has cultivated a reputation for utter forgettability. She never speaks out of turn (in public), never has a daring thought (that she admits aloud), and never comes close to courting scandal...as far as anybody knows.
But years ago, her path crossed Robert's and she's never forgotten the extraordinary lengths he travelled to keep her safe when she hadn't a friend in the world. She longs to be his demure duchess... but little does he know that to marry her would be utter madness.