Free Fall by Laura Anne Gilman

The Cosa Nostradamus is in disarray; the Truce holding violence in check has been broken. Magical Manhattan is at war, and Wren Valere is left without her partner/lover Sergei, whose past loyalties keep him from her side just when she needs him the most.

Hoping to keep herself occupied, Wren takes a job— but what should have been an ordinary Retrieval instead forces her to realize that it is time to do more. It is time for the Cosa to take the battle to the enemy.

But she'll do it her way. The Retrievers' way. Sneaky, smart—and with maximum damage possible. What was lost will be found, what was stolen will be Retrieved. And this war will be ended, once and for all.

Wren Valere always finishes the job. Always.