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!