The caption is silly. The notion that Obama is the most liberal candidate to ever run is not based on anything objective. Reading is not a "liberal" thing. it is a knowledge thing.
"Reading is to the mind what exercise is to the body." -- Sir Richard Steele
It's amazing how some folks get caught up in a couple of smear rumors that have been long and widely discredited but are still apparently oblivious of the issues that matter.
"The great enemy of the truth is very often not the lie -- deliberate, contrived and dishonest, but the myth, persistent, persuasive, and unrealistic. Belief in myths allows the comfort of opinion without the discomfort of thought." --John F. Kennedy
Because it's there, obviously. Well-read people choose a broad assortment of books to get a broad perspective. People are fed up with shallow, uninformed presidents wearing blinders.
"If you know yourself but not your enemy, for every victory gained you will also suffer a defeat." -- Sun Tzu