Even though it is more than ten years old now, Moret & Shapiro is still the best book available on NP completeness and related topics. In a way I disagree with other reveiwers who advise that only graduate students read this book. It all depends. I have known a number of graduate students who might not have understood this material as well as some undergraduate students. It is not so much a matter of college level as it is your personal tools for dealing with abstraction and complexity.