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John Locke's Two Treatises of Government are essential to understanding social contract theory as it applies to free individuals.
Multiple copies of the Two Treatises may be found at the Internet Archive. The second treatise may be read for free, online at Project Gutenberg.
Websites and are for popularizing and critiquing Locke's Second Treatise on Government and John Rawls's A Theory of Justice, with a goal of relegating Jeremy Bentham's disastrous Utilitarianism to the dust-bin of Anglo-American legal theories.