Everything Plantinga has ever written is worth reading. His warrant trilogy contains the majority of his work in epistemology (he has a few scattered articles and book chapters, but the essential reading is in his trilogy). Warrant: The Current Debate is an extended critique of all the main views at the time (1993) about warrant, "that quality enough of which turns true belief into knowledge." Warrant and Proper Function then presents his own account of warrant. Both of these books are very interesting, the latter being more important for understanding his own view, but the former showing why he thinks the other views don't work. Finally, Warranted Christian Belief extends his account of warrant to theistic and explicitly Christian beliefs, showing that if Christianity is true, then Christians are warranted in believing Christian doctrine (i.e., Christians know that Christianity is true). This is an absolute gem of a book, and along the way Plantinga discusses in detail Freud and Marx, religious pluralism, the problem of evil, biblical higher criticism, and a number of other fascinating subjects (and his comments on fundamentalism are perhaps the funniest I've ever seen in a philosophical work). Highly recommended.