The Governmental Process: Political Interests and Public Opinion

This is probably the best reference book for any political science student interested in Group Theory. I would also recommend work by V.O. Key and Robert Dahl, but Truman's work has been the most useful to me. It was first written in 1951 but the most modern books on the subject still refer to this book. It's best quality is its completeness. I have about 23 books at my feet write now as I work on a paper for my Masters Degree, but I find myself picking up Governmental Process more than any other.