Keith Laumer's Worlds of the Imperium is the first in a series of science fiction, alternate universe novels.
American diplomat Brion Bayard is kidnapped by agents of the Imperium and taken to their version of Earth. This alternate Earth's technology and society in general are roughly equivalent to Europe around World War I, but they have, by fortuitous accident, invented a means of traveling between universes. Apparently so has somebody else. There have been raids by an unknown enemy, one who hasn't hesitated to detonate a nuclear bomb in a recent attack.
There are only three universes in which humanity hasn't destroyed itself: the Imperium, Bayard's and a third which is thought to be the home of the attackers. The Imperium wants Bayard to secretly kill the Brion Bayard in that war-torn Earth and take his place as its ruler. After intense training, he is sent in. However, Bayard is easily unmasked (the "real" Bayard lost his legs in combat). He is surprised to be warmly greeted by the other Bayard, but then the latter is assassinated.
It turns out that the attacks are being staged by a secret organization within the Imperium itself in a bid to seize power. Bayard manages to foil their scheme. Offered a chance to return home as a reward, he instead chooses to remain in the service of the Imperium.
Best part of story, including ending:
Laumer has a talent for fast-paced adventure stories, and this one is no exception.
Best scene in story:
The scenes showing life in the Imperium, which is presented as a sort of idealized version of early 20th century European society, complete with gaudy uniforms and codes of honor.
Opinion about the main character:
Bayard is resourceful and cool under pressure.