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.
British Library: Taking Liberties - Hobbes' Leviathan. (Archived).
The Guardian: The 100 best nonfiction books: No 94 – Leviathan by Thomas Hobbes (1651), by Robert McCrum, 20 Nov 2017.
Internet Encyclopedia of Philosophy: Hobbes, Thomas - Moral and Political Philosophy, by Gareth Williams.
Librivox: Leviathan - public domain audiobook.
Online Library of Liberty: Oakeshott’s Introduction to Leviathan. HRML format.
PhilPapers: Thomas Hobbes - bibliography with open access option.
Stanford Encyclopedia of Philosophy: Hobbes's Moral and Political Philosophy, by Sharon A. Lloyd, Susanne Sreedhar.
Wikipedia: Leviathan (Hobbes book)
Further reading at Tom's Learning Notes
Spinoza: Theologico-Political Treatise
Locke: Two Treatises of Government
John Aubrey: Brief Lives - includes a life of Hobbes.
Authors referred to in Leviathan
Virgil: The Aeneid
Josephus: Against Apion
Ammianus Marcellinus: Roman History
St Jerome: Vulgate Bible
Bede: Ecclesiastical History
Francisco De Suarez: De Concurso, Motione et Auxilio Dei
Edward Coke: Institutes
Robert Bellarmine: Disputationes
John Selden: On Titles of Honour