Theological works and Mysteries, Films, Novels, Architecture, Economics, Psychoanalysis, Continental Philosophy, History of Communism, History of Democracy, Globalization, Colonialization, Twentieth Century History, Cultural Phenomena, News Papers, Magazines, Websites and Blogs, essentially everything of significance in the Western Tradition and World cultural history and political history.
In other words Zizek has it seems read everything. And the more of everything you have read yourself the more you will get out of his commentary on everything.
Probably reading widely in disparate subjects is the key to understanding what Zizek is saying. He knows it, and he speaks from knowing it. And he expects you to know it. In other words he sets the bar high for his audience. He expects them to know basically what he knows in order to be in dialogue with him. If you don’t know the material you might think he is winging it. He is not winging it. He knows it and he is telling you that you need a really long reading list if you are going to understand much of what is happening in our age. But having just read it is not enough. You need to also have read in order of precedence Hegel, Lacan, Badiou, Derrida, Deleuze, Foucault as well as Marx, Lenin, Trotsky, etc. Economists, Political Thinkers, Sociologists and other commentators on Political, Social, Economic, and Historical Phenomena.
In other words Zizek is an educated European with wild ideas, a love of shocking people and a gift for gab who always makes salient theoretical points, even when asked for sound bites. To keep up you are going to have to read more than Zizek.