A Gentleman of Unreliable Honor by Grace Burrowes

He’s cheated death…
Lord Julian Caldicott is summoned by his estranged mother to search for old love letters she has misplaced. Julian soon realizes the letters along with other items of sentimental value have been stolen. The thief is preying on wealthy women, purloining mementos both dear and scandalous. If the missing items become public, many reputations will be ruined, including the duchess’s.

…but dishonor might yet defeat him.
Julian embarks on the investigation despite having no clues, too many suspects, and his distant and difficult mama at the center of the whole intrigue. In the midst of these frustrations, Julian’s ducal brother takes ship for France, adding more duties to his lordship’s already full plate. Before Julian can bring the culprit behind the thefts to justice, he must first come to terms with old betrayals that could yield new and deadly fruit.