Keith Laumer 
Worlds of the Imperium 

American Diplomat, Brion Bayard, is on assignment in Stockholm, Sweden, when he is kidnaped by agents of The Imperium who are from a parallel world. A world where the American revolution never happened and the British Empire, which includes America and the German and Austro-Hungarian empires of Europe rule the world. Bayard learns that he was abducted because the Imperium is under attack from another parallel world. A parallel world ruled by a ruthless dictator who happens to be. . . Brion Bayard. It up to Brion to kill his alternate self and step into his place to stop these attacks. But Brion discovers almost immediately that nothing is as it seems.