amazon.co.uk or amazon.com) - it's mosty about science as a 'candle in the dark' of illogical beliefs. At the end he speaks out powerfully on the subject of free speech. It strikes me both with the Simon Singh libel case (now, thankfully, decided in Simon's favour) and all the things we're not allowed to say, in case it causes offense or incites people to riot, that we're heading very much in the wrong direction.
Sagan points out that 'within certain narrowly circumscribed limits - Justice Oliver Wendell Holmes's famous example was causing panic by falsely crying "Fire!" in a crowded theatre - great liberties are permitted in America.' He cites examples like burning effigies of the President, devil worship, a purported scientific article or popular book asserting the 'superiority' of one race, praising people like Hitler and arguing that religious groups (or masons) are plotting to take over the world.
He refers to John Stuart Mill from On Liberty, saying that silencing an opinion is a particular evil. If the opinion is right, we are robbed of the opportunity of exchanging error for truth, and if it's wrong, we are deprived of a deeper understanding of the truth 'in its collision with error.' If we know only our side of the argument, we hardly know even that; it becomes stale, soon learned only by rote, untested, a pale and lifeless truth.
Perhaps the strongest argument was from that greatest of American founding fathers, Thomas Jefferson. He's quoted as writing 'A society that will trade a little liberty for a little order will lose both, and deserve neither.'