Fear Itself by Rosenheim, Andrew;

Long description:

It's the late 1930s in an America slowly pulling itself out of the Depression. War is threatening in Europe, but in America, with forty million citizens of German ancestry, there is great pressure to stay out of the fight.

Jimmy Nessheim, a young Special Agent in the fledgling FBI, is assigned to infiltrate a new German-American organisation known as the Bund. Ardently pro-Nazi, it is conspiring to sabotage President Roosevelt's efforts to stop Hitler's advance. But as Nessheim's investigation takes him into the very heart of the Bund, it becomes increasingly clear that something far more sinister is at work, something that seems to lead directly to the White House. Drawn into the centre of Washington's high society, Nessheim finds himself caught up in a web of political intrigue and secret lives. But as he moves closer to the truth, an even more lethal plot emerges, one that could rewrite history in the most catastrophic of ways...

Set in the tense years before the Second World War, Fear Itself offers a rich depiction of history as it was - and as it might have been. A compelling thriller, it tells the riveting story of a plot that had the potential to change the world