The Rhetoric (Greek: Ῥητορική; Latin: Ars Rhetorica) by Aristotle is a treatise on the art of persuasion, examining how a public speaker can produce a range of effects, including a favourable impression of his own character, and various emotions, as well as winning assent to arguments. As so often with Aristotle, the Rhetoric was foundational for the discipline, setting the agenda down to early modern times.
Free online texts
Internet Archive: The Rhetoric of Aristotle, translated by Richard Claverhouse Jebb (1908). Multiple formats.
Internet Classics Archive: The Rhetoric, translated by W. Rhys Roberts. HTML and TXT formats.
University of Adelaide (Internet Archive): Rhetoric, translated by W. Rhys Roberts. EPUB, HTML and MOBI formats.
Wikisource: Rhetoric, multiple translations, multiple formats.
History of Philosophy without any gaps: Stage Directions: Aristotle's Rhetoric and Poetics - podcast by philosopher Peter Adamson.
Librivox: Rhetoric, public domain audiobook.
Stanford Encyclopedia of Philosophy: Aristotle's Rhetoric, by Christof Rapp.
The Great Conversation: Further reading at Tom's Learning Notes
Aristotle: The Poetics - his other significant work on aesthetics.
Ancient Greek resources: Learn to read Greek classics in the original.