Some swear by 'The Wealth of Nations' or 'On Liberty' or even 'Das Kapital.' My text is 'Rerum Novarum.'
That, and the successor encyclicals, 'Quadragesimo Anno,' 'Mater et Magistra,' and 'Centesimus annus,' do pretty much correspond with my own social views.
'Rerum Novarum' was issued in 1891 by Pope Leo XIII. It became the basis for the Distributist movement and for Catholic social, economic and political thought in general. The encyclical argues for responsibility and balance, condemning both the extremes of capitalism and socialism. The other three missives follow and update its lead.
It should be noted that when Leo condemns 'Liberalism,' he is using it in the 19th Century sense -- libertarian laissez faire capitalism. Similarly, when these encyclicals speak out against Socialism, they mean 'Real Socialism,' as John Paul II puts it, state ownership of the means of production. Not the welfare state (although there are warnings against its abuses).
At any rate, I decided to put together an ebook of these four encyclicals, free to anyone who wishes to download it. There are both EPUB and PDF versions available at my website.