Clive Cussler's The Sea Wolves. Isaac Bell #13

Can one detective stop a war being lost just as it starts?August, 1914, the eve of WWI, and in New York the Van Dorn Detective Agency is supervising the loading of a final shipment of rifles bound for Britain. When Chief Investigator Isaac Bell discovers a hidden radio transmitter, he realises it will broadcast the exact location of the merchant ship to waiting German U-boats. This ship is saved ­- but two others have already departed.
In a desperate race against time, Bell sets out to foil these acts of sabotage. Yet even if he succeeds, he knows he must hunt down the saboteur. Little does he realise that a sophisticated German spy ring is operating out of New York City's docks - and they've a secret that could determine the outcome of the war .