Riley-Smith's 1986 book gives convincing case for a 'revisionist' view of the crusades, challenging the common belief that the crusades were motivated by fanaticism and were designed to plunder the Holy Lands.

Butler's 1990 work shook the foundations of feminist theory and changed the conversation about gender. While many thinkers already accepted that "gender" was a category constructed by society defined by one's genitalia, Butler went further and argued that gender is performative-it exists only in the acts that express it.

Capitalism, thought Karl Marx, works by exploiting the working class. Their wages do not reflect the value of their labor. Marx concluded that capitalism would fail because of this contradiction at the heart of the capitalist system. He wrote Capital to give activists the theories and language they needed to criticise the system.

Theory of International Politics created a "scientific revolution" in international relations, starting two major debates. It defined the 1980s controversy between the 'neorealists,' who believed that competition between states was inevitable, and the 'neoliberals,' who believed that states could co-operate.

In Philosophical Investigations, Ludwig Wittgenstein presents a radical approach to the philosophy of language and the mind, setting out a startlingly fresh conception of philosophy itself. Wittgenstein begins from the insight that most philosophical problems trace back to incorrect assumptions about the nature of language.

Though written more than 500 years ago, Niccolo Machiavelli's The Prince is still both widely read and very influential. Readers turn to it for its direct advice on the question of how to attain - and retain - power. Machiavelli's answer, in brief: use any means necessary to make sure the state survives.

Considered his most important work, Mahbub ul Haq's Reflections on Human Development appeared at the end of his career in international development, and consolidates his revolutionary contribution to the discipline.

In Citizen and Subject, Mahmood Mamdani challenges dominant views of the crisis of postcolonial Africa, particularly that the problems the continent faces are home grown. Citizen and Subject insists that the current crisis is the institutional legacy of colonialism.