I highly recommend Lies My Teacher Told Me by James W. Loewen. It'll give you an excellent background on American history as well as a lot of insight that explains why our political system works (or doesn't work) the way it does.
For irreverant reading, I agree that you've got to pick up P.J. O'rourke.
I'm also a huge Hunter S. Thompson fan. His book Fear and Loathing on the Campaign Trail '72 made me become a journalist.
Now, I'm not a Democrat, and a lot of what Al Franken writes is pro-Democratic party, but he really does an excellent job of exposing current political conniving. It's somewhat discouraging, but The Truth with Jokes is an enlightening book.
After that, you may need a really good laugh, because there aren't really that many jokes in Al Franken's last book. So, I really love the Daily Show's book America. It's wicked.