The Academica is Cicero's main philosophical work on the theory of knowledge. The first edition, the Academica Priora, consisted of two books, the dialogues Catulus and Lucullus, of which only the latter is extant. Lucullus defends the stoic position on the possibility of certain knowledge, which Cicero argues takes the view of the academic sceptics that it is necessary to accept what is merely probable.
Part of a revised version, the Academica Posteriora, in which Varro replaced Lucullus as the main interlocutor, also survives.
Free online texts
Gutenberg: Academica. Multiple formats.
Gutenberg: The Academic Questions, Treatise De Finibus, and Tusculan Disputations. Multiple formats.
Internet Archive: De natura deorum; Academica; with an English translation by H. Rackham (1933). Multiple formats.
Internet Archive: The Academica of Cicero. Latin text, edited by James Smith Reid. Multiple formats.
University of Adelaide (Internet Archive): The Academic Questions, translated by Charles Duke Yonge. EPUB, HTML and MOBI format.
Wikisource: Academica Priora - Latin text. Multiple formats.
History of Philosophy without any gaps: Rhetorical Questions: Cicero - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Raphael Woolf on Cicero - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Cicero: Academic Skepticism, by Harald Thorsrud.
The Great Conversation: Further reading at Tom's Learning Notes
Sextus Empiricus: Outlines of Pyrrhonism - main source for the Pyrrhonist school which propounded a rival form of scepticism to that of the Academics.
Latin resources: Learn to read Latin texts in the original.