Agent 6 by Tom Robb Smith (2012) Grand Central Publishing

The third book in the Child 44 trilogy.  Leo Demidov’s wife and two adopted daughters are invited to travel to New York City, but Leo is forbidden to travel.  This ends in tragedy and Leo’s request to investigate is denied.  So he goes after agent 6 in a roundabout fashion, first travelling to Afghanistan, and eventually to the streets of New York.