The American Congress: The Building of Democracy by Julian E. Zelizer. I highly recommend this book because it takes historians specializing in each aspect of Congress/period of Congressional History and compiles their work into a book that helps the reader get an amazing insight into how Congress functions, both in the past and currently.

The recent book "The First Congress" by Fergus Bordewich focuses on the beginnings of Congress, and how the very first two sessions of Congress set up the structure of the U.S. Government (and wrote the Bill of Rights along the way) without any precedent or compass to guide them in that huge task.