It depends on how much it focused on either topic, but Empire Politics and Economy should be sorted under Politics. You can talk about the economy as a side topic of a study on a nation's politics and policies. But books on economy generally would not have much emphasis on politics as there are too many variables to factor into which may include history, geography, social constructs, foreign relations, famine and population growths for example, and so politics would be a mere side note of a book on economy.
As for sorting based on author, index cards is the lowest tech solution.