If you want a book that does a good job of explaining recursion in simple terms, take a look at Gödel, Escher, Bach: An Eternal Golden Braid by Douglas Hofstadter, specifically Chapter 5. In addition to recursion it does a nice job of explaining a number of complex concepts in computer science and math in an understandable way, with one explanation building on another. If you haven't had much exposure to these sorts of concepts before, it can be a pretty mindblowing book.
