A Gentleman in Possession of Secrets by Grace Burrowes

Somebody has made off with the loot–and with the lady!
An acquaintance from Lord Julian’s military years begs his lordship’s help locating the fellow’s missing fiancée. The young lady’s family has only the flimsiest explanation for her disappearance, and the harder Julian digs for answers, the more complicated the puzzle becomes. He soon realizes that a young woman who was described as a bluestocking of no particular fortune might in fact have absconded with a hoard of gold, or been kidnapped in hopes she’d reveal the location of a literal treasure.

Julian must sort through lies, scandal, family feuds, and conflicting clues, all while battling his own demons, or the course of true love might end in doom.