Leviathan by Thomas Hobbes is one of the outstanding works of political theory in the English language. It develops an early social contract theory to argue that human beings must put themselves under a single absolute sovereign as the only way out of a state of nature in which life is famously said to be 'nasty, brutish and short'.
For much of Hobbes' life such an argument would have been seen as favourable to the Stuart monarchy. By the time of Leviathan's publication in 1651, however, it could be read as supporting the effective authority of Cromwell's Commonwealth. Hobbes was acordingly repudiated by former associates among the exiled royalist court in Paris.
His time in France was nevertheless fruitful through associations with the leading philosophers and scientists of the day. This is reflected in the mechanical philosophy of the opening pages of Leviathan, in which living things are compared to automata, and the state to an artificial man, a figure memorably illustrated in the book's original frontispiece.
Free online texts
Early Modern Texts: Leviathan, adapted and translated into modern English, by Jonathan Bennett. PDF format.
Online Library of Liberty: Leviathan, edited by W.G. Pogson Smith (1909). Multiple formats.
Marxists.org: Leviathan (first five chapters) - from the Cambridge Revised Student Edition, Edited by Richard Tuck, 1996. HTML format.
University of Adelaide (Internet Archive): Leviathan. Multiple formats.
Wikisource: Leviathan. HTML and other formats.