In addition, here are some good websites that carry free versions of many public domain works:
- The Internet Classics at MIT has 441 classics available, mostly Western but with a handful of non-Western works as well.
- Library of Economics and Liberty
- The Online Library of Liberty has many works and includes essays about them and biographies of the authors. This appears to be an excellent site.
- The Mises Institute focuses on the Austrian school of economics, but seems to have a good online library available in various digital formats. It seems to have scholarly introductions to the works as well.
- Perseus Digital Library at Tufts University has online collections for ancient Greek and Roman history, including an art and archaeology artifact viewer; Arabic materials; Germanic materials; 19th century American history; Renaissance materials; the Richmond Times Dispatch; and Humanist and Renaissance Italian poetry in Latin.
- Project Gutenberg (of course)
Plato (and Socrates via Plato)
In addition to the above, I would like to recommend a book Grim linked some time ago, Mortimer J. Adler's Aristotle for Everybody: Difficult Thought Made Easy. It is an introduction to Aristotle's thought written for his sons when they were 11 and 13. Written with simple language and examples, it deals with some very deep subjects in a way I've found very accessible. That said, the first two chapters seemed too simple to me; I almost stopped reading because it seemed over-simplified and I wondered if I would get anything out of it. After that, however, it picks up and I'm glad I kept on.
Tao Te Ching (Wikipedia), Lao Tzu (In pinyin, these are Daodejing and Laozi. The old Wade-Giles spelling is how they were first introduced in English, and it has stuck for many.)
The Art of War (Wikipedia), Sun Tzu
There are also some important medieval works, but they appear to be rather inaccessible without some amount of preparation. I'm afraid I cannot do justice to them here at this time, but I would like to offer the following links as possible starting points.
Grim suggests Aquinas as a starting point, so:
Thomas Aquinas's page at Wikipedia gives a brief overview of his life and thought, and includes good links to a number of other resources.
The Stanford Encyclopedia of Philosophy has a number of entries on Aquinas, but the page on the man himself is probably the best starting point at the SEP for his philosophy (that I have seen, anyway). It is a lengthy article that ends with a bibliography that includes 10 different introductions to Aquinas, numerous other works about him, links to his own works in Latin and English, and more.
A philosophy course at Vasser University offers this 5-page guide to reading Aquinas's Summa Theologiae, which seems useful.
Here is a highly-hyperlinked version of the Summa Theologiae at New Advent. (The top of the page is an ad to buy the paper version, but you can read it online there for free.)
Although not free, Grim also recommends Sidney Painter's Feudalism and Liberty, which he discussed in a post in 2010.
Some additional sources he offers to get started, with the caveat that they have weaknesses in the very area we are trying to work with, include Anthony Kenny's Medieval Philosophy and the Stanford Encyclopedia of Philosophy article on Medieval Political Philosophy.
Second Treatise of Civil Government (1689) (Wikipedia), John Locke (The link goes to the second of Locke's treatises, but the website has put them both on the same page, so you should land about halfway down the page. The top of the page has some good commentary, and here is the beginning of the first treatise, for the curious.)
The Law of Nations (1758) (Wikipedia), Emerich de Vattel
The Social Contract (1761) (Wikipedia), Jean-Jacques Rousseau
An Inquiry into the Nature and Causes of the Wealth of Nations (AKA, The Wealth of Nations) (1776) (Wikipedia), Adam Smith
The Federalist Papers (1787-1788) (Wikipedia), Alexander Hamilton, John Jay, and James Madison
(If you want to download them, the Library of Congress recommends the files at Project Gutenberg)
The Law (1850), Frederic Bastiat. There is no Wikipedia article on this work, but it is only about 55 pages long.
The Progressive Era
The Promise of American Life (1909), Herbert Croly.
If you have any other public domain works that you think I should add to this post, please let me know in the comments. Also, this was a lot of copying and pasting. I've tried to get it all right, but if you notice any errors, bad links, etc., please let me know.
I have tried to go through all the comments in the Common Ground: Sources thread and pick out all of the ones that seemed to fit here, but if I have missed one of yours, please let me know so I can add it.
If you are interested in the rest of the Common Ground series, I'm tagging them all with that term. You can click the tag on any of these posts to get the whole series.
UPDATE 2/10/16: I have added sources suggested in the comments up to this date.