Ethics, Demonstrated in Geometrical Order (Latin: Ethica, ordine geometrico demonstrata), is a Latin philosophical treatise by Baruch Spinoza, written in around 1664-5 and published shortly after Spinoza's death in 1677.
Spinoza presents his argument as a series of propositions, derived logically from a small number of axioms after the manner of Euclidean geometry. His system is monistic, in that posits reality as consisting of a single substance, and pantheistic in that identifies this substance with God.
Spinoza's conception of God is, however, sufficiently abstract that his thought has often been equated with atheism. This was for a long time a source of huge opprobrium, which meant that his influence on the radical enlightment of the seventeenth and eightenth centuries was largely subterranean. Since then, influence in his work has consistently grown. Among his twentieth century admirers was Albert Einstein who once said that he believed in Spinoza's God 'who reveals himself in the lawful harmony of the world, not in a God who concerns himself with the fate and the doings of mankind.'
Free online texts
Bibliotheca Augustana: Ethica Ordine Geometrica Demonstrata. Latin text, HTML format.
Early Modern Texts: Ethics Demonstrated in Geometrical Order, adapted and translated into modern English, by Jonathan Bennett. PDF format.
EthicaDB: The Ethics. Translations in multiple European languages. HTML format.
Gutenberg: Ethics, translated by R.H.M. Elwes. Multiple formats.
Internet Archive: Improvement of the Understanding, Ethics and Correspondence, translated by R.H.M. Elwes. Multiple formats.
Middle Tennessee State University: Ethics, translated by RH.M. Elwes. HTML format.
SpinozaBase: Ethica. Latin text. HTML format.
BBC Radio Great Lives: Benedict Spinoza. With Humphrey Carpenter and Sir Harry Kroto.
BBC Radio 4 In Our Time: Spinoza. Melvyn Bragg with Jonathan Rée, Sarah Hutton and John Cottingham.
Librivox: The Ethics - public domain audiobooks.
Marxists.org: Ethics. Short extracts from the opening pages of each of the first four parts. 1989 G.H.R. Parkinson translation. HTML format.
Marxists.org: Hegel's Lectures on the History of Philosophy - Spinoza.
Marxists.org: Ilyenkov - The History of Dialectics - Spinoza.
Internet Archive: Guide to Spinoza's Ethics, by Beth Lord.
PhilPapers: Spinoza - bibliography with open source option.
Spinoza Gesellschaft: Spinoza Bibliography.
Further reading at Tom's Learning Notes
Aristotle: The Nicomachean Ethics.
Thomas Hobbes: Leviathan - Hobbes' materialistic worldview has some similarities with Spinoza's monism.
Latin Resources: Online materials for learning Latin.