The Leader is a 2003 alternative history of World War II by Guy Walters.
The book's premise is that King Edward VIII does not abdicate but reigns as King with Wallis Simpson as Queen, eventually appointing Oswald Mosley as a dictatorial Prime Minister. The resulting fascist Britain is allied with a victorious Adolf Hitler. This regime is opposed by the hero of the book, Captain James Armstrong.
The book is distinguished from some other alternate histories - like Robert Harris's "Fatherland" - which reach a similar result from a military defeat and conquest of Britain, with the Nazis restoring an already abdicated Edward VIII to the throne of the occupied country.