The world's best introduction to epistemology,
Understanding Knowledge
discusses basic philosophical problems in epistemology, including such questions as: What is knowledge? How can we know about the world outside our minds? How do we know about things that are unobservable? Are intuitions a legitimate source of knowledge? What justifies any of our beliefs? The text clearly and succinctly explains the most important theories and arguments about these things, and it does so a lot less boringly than most books written by professors.