A real good one to start with is "What Liberal Media" by Eric Alterman. (At about this time Pat will be jumping in comparing this book to those two terribly researched ones from the right, "Slander" and "Bias", however, the research of Alterman's book is more thorough. It's like saying Richard Marx' music is on the same par with DB's).

Also, pick up some copies of Extra! magazine and do a search for Eric Cohen at Amazon. He's also written a couple of books on the media. The titles escape me.

Finally, just sit down and list all the punditry on tv and radio, then list them in two columns and you'll clearly see that the true left is left out in the cold.