Imperialism and Social Reform: English Social-Imperial Thought, 1895–1914 (1960).
Jamaican Blood and Victorian Conscience: The Governor Eyre Controversy (1962). Alternative titles: "The Governor Eyre Controversy" (UK edition); "Democracy vs. Empire" (Anchor Doubleday paperback edition).
Occasional Papers of T. R. Malthus (edited with an introduction, 1963).
The Rise of Free Trade Imperialism: Classical Political Economy, the Empire of Free Trade and Imperialism, 1750–1850 (1970).
Élie Halévy, The Birth of Methodism (translated and with an introduction by Semmel, 1971).
The Methodist Revolution (1973).
Marxism and the Science of War (1981).
John Stuart Mill and the Pursuit of Virtue (1984).
Liberalism and Naval Strategy: Ideology, Interest and Sea Power during the Pax Britannica (1986).
The Liberal Ideal and the Demons of Empire: Theories of Imperialism from Adam Smith to Lenin (1993).
George Eliot and the Politics of National Inheritance (1994).