Basic Techniques of Combinatorial Theory

This classic (1978) textbook is 30 years old, but still very useful and relevent. It covers the basic undergraduate course in combinatorial logic, theory, and practice; nobody has ever done it better. Besides a clear and easy-to-understand exposition, this book has some of the best problem sets that I have ever seen. There are, I admit, some more advanced books with recent results. But if you really want to understand this subject, and if you are willing to work through hundreds of excellent problems, I can guarantee you that Cohen is your man!