The Twelve Thieves of Christmas - Chronicles of a Lady Detective, #5

A Christmas bounty of murderers and thieves. Pinkerton detective Penelope Hamilton is happiest when she's working on a case. This Christmas season of 1887 should be very merry indeed, as she has two. One seems simple enough-a surveillance job for her estranged husband, Frank, a fellow Pinkerton. The other promises to be much more interesting. How did a society matron's valuable necklace, lent a year before to a friend who was then murdered by a thief, turn up around the neck of a young debutante from Chicago's high society? All too soon, Pen's inquiries stir up trouble, and now a young mother and child are in harm's way, an assassin is stalking Frank, and a killer claims another victim.
Pen needs more than her trusty lockpicks and derringer this time, so she turns to allies old and new, including an impulsive young society girl, a snoopy housemaid (who's surprisingly good at knots) and, most of all, a distractingly handsome and supposedly reformed jewel thief.  One thing is clear-she's going to need a Christmas miracle to save them all.