This list first appeared in the May 3, 1999, issue of National Review, and is conveniently available here. The first 15 follow. I doubt they are listed in order of importance.
1. The Second World War, Winston S. Churchill
Brookhiser: "The big story of the century, told by its major hero."
2. The Gulag Archipelago, Aleksandr I. Solzhenitsyn
Neuhaus: "Marked the absolute final turning point beyond which nobody could deny the evil of the Evil Empire."
3. Homage to Catalonia, George Orwell
Herman: "Orwell's masterpiece-far superior to Animal Farm and 1984. No education in the meaning of the 20th century is complete without it."
4. The Road to Serfdom, F. A. von Hayek
Helprin: "Shatters the myth that the totalitarianisms 'of the Left' and 'of the Right' stem from differing impulses."
5. Collected Essays, George Orwell
King: "Every conservative's favorite liberal and every liberal's favorite conservative. This book has no enemies."
6. The Open Society and Its Enemies, Karl Popper
Herman: "The best work on political philosophy in the 20th century. Exposes totalitarianism's roots in Plato, Hegel, and Marx."
7. The Abolition of Man, C. S. Lewis
Brookhiser: "How modern philosophies drain meaning and the sacred from our lives."
8. Revolt of the Masses, Jose Ortega y Gasset
Gilder: "Prophesied the 20th century's debauchery of democracy and science, the barbarism of the specialist, and the inevitable fatuity of public opinion. Explained the genius of capitalist elites."
9. The Constitution of Liberty, F. A. von Hayek
O'Sullivan: "A great re-statement for this century of classical liberalism by its greatest modern exponent."
10. Capitalism and Freedom, Milton Friedman
11. Modern Times, Paul Johnson
Herman: "Huge impact outside the academy, dreaded and ignored inside it."
12. Rationalism in Politics, Michael Oakeshott
Herman: "Oakeshott is the 20th century's Edmund Burke."
13. Capitalism, Socialism, and Democracy, Joseph A. Schumpeter
Caldwell: "Locus classicus for the observation that democratic capitalism undermines itself through its very success."
14. Economy and Society, Max Weber
Lind: "Weber made permanent contributions to the understanding of society with his discussions of comparative religion, bureaucracy, charisma, and the distinctions among status, class, and party."
15. The Origins of Totalitarianism, Hannah Arendt
Caldwell: "Through Nazism and Stalinism, looks at almost every pernicious trend in the last century's politics with stunning subtlety." (more)