In the media, they use the word "conservative" 20 times more than they use the word "liberal". Nothing like this has happened since the 60s, where they would say "A black robber...."
Well, as that book proved, if there was a left bias, there'd be more favorable news stories towards left candidates.
In the period that Al Franken and his staff looked at for the book (I cant remember exactly, but I think it was Bush v Gore election) there were more pro right candidate stories than pro left.
(Al Franken, Jon Stewart, Steven Colbert... it says a lot about the current state of the USA where you have to look to Comedians for truth....)