Sources of the American Enlightenment are many and vary according to time and place. As a result of an extensive book trade with Great Britain, the colonies were well acquainted with European literature almost contemporaneously. Early influences were English writers, including James Harrington, Algernon Sidney, the Viscount Bolingbroke, John Trenchard and Thomas Gordon (especially the two's Cato's Letters), and Joseph Addison (whose tragedy Cato was extremely popular). A particularly important English legal writer was Sir William Blackstone, whose "Commentaries on the Laws of England" served as a major influence on the American Founders and is a key source in the development Anglo-American common law. Although John Locke's "Two Treatises of Government" has long been cited as a major influence on American thinkers, historians David Lundberg and Henry F. May demonstrate that Locke's "Essay Concerning Human Understanding" was far more widely read than were his political "Treatises".