Edgar Rice Burroughs 
Mad King 

Set in the fictional European kingdom of Lutha, the protagonist is a young American named Barney Custer, of Beatrice, Nebraska, who is the son of an American farmer and a runaway Luthan princess, Victoria Rubinroth. Unaware of his royal blood, much less that he is a dead ringer for his relative Leopold, the current king of Lutha, Barney visits Lutha on the eve of the First World War to see for himself his mother’s native land. As he arrives in Lutha, King Leopold has just escaped from his ten years’ imprisonment at the hands of his scheming uncle, Prince Peter of Blentz. Much to his own and everyone else’s confusion, Barney is naturally mistaken for the king, leading to numerous complications. (Wikipedia)