The Politics of Aristotle is, like many of his works, the oldest extant treatise on the subject, although it allude to, and sometimes criticizes, the discussion of political issues in Platonic dialogues like the Republic.
Book One opens with the famous declaration that the human being is by nature a political animal, and includes discussions of slavery and of economics that are still of significance for contemporary debates, in part because of their distance from modern views. Book Two looks at the best form of government, critiquing philosophical proposals including those of Plato in The Republic and The Laws. Book Three provides a classification of constitutions, something that Plato had already undertaken in The Republic. Books Four to Six look at the multiplicity of existing constitutions and the changes to which they are subject, while Books Seven and Eight return to the subject of the ideal constitution and the form of education appropriate to it.
Free online and downloadable texts
Gutenberg: Politics, translated by William Ellis, 1912. Multiple formats.
Internet Classics Archive: The Politics, translated by Benjamin Jowett. Online text and downloadable. .txt file.
University of Adelaide (Internet Archive): The Politics, translated by Benjamin Jowett. EPUB, HTML and MOBI formats.
BBC In Our Time: Aristotle's Politics - archived radio discussion with Melvyn Bragg.
History of Philosophy without any gaps: Constitutional Conventions: Aristotle's Political Philosophy - podcast by philosopher Peter Adamson.
Archive for the History of Economic Thought: Aristotle as Sociologist, by Charles A. Ellwood, 1902.
Internet Encyclopedia of Philosophy: Aristotle: Politics, by Edward Clayton.
Librivox: Politics - public domain audiobook.
Stanford Encylopedia of Philosophy: Aristotle's Political Theory, by Fred Miller.
The Great Conversation: Further reading at Tom's Learning Notes
Ancient Greek resources: Learn to read Greek classics in the original.