Any Old Diamonds by K.J. Charles

Lord Alexander Pyne-ffoulkes is the younger son of the Duke of Ilvar, with a bitter grudge against his wealthy father. The Duke intends to give his Duchess a priceless diamond parure on their wedding anniversary—so Alec hires a pair of jewel thieves to steal it.

The Duke’s remote castle is a difficult target, and Alec needs a way to get the thieves in. Soldier-turned-criminal Jerry Crozier has the answer: he’ll pose as a Society gentleman and become Alec’s new best friend.

But Jerry is a dangerous man: controlling, remote, and devastating. He effortlessly teases out the lonely young nobleman’s most secret desires, and soon he’s got Alec in his bed—and the palm of his hand.

Or maybe not. Because as the plot thickens, betrayals, secrets, new loves, and old evils come to light. Now the jewel thief and the aristocrat must keep up the pretence, find their way through a maze of privilege and deceit, and confront the truth of what’s between them…all without getting caught.