Thyng that I speke, it moot be bare and pleyn.
I sleep nevere on the Mount of Pernaso,
Ne lerned Marcus Tullius Scithero.
Marcus Tullius Cicero (106 BC - 46 BC) is cited by Chaucer as an expert on rhetoric (mainly because of the Rhetorica ad Herennium, which he may not have written) and (in the Melibee) as a source of wise sententiae, drawn from such essays as De senectute. His major works are:
De officiis, tr. Walter Miller, Loeb, Cambridge, MA. 1913 [PA 6296.D5].
De senectute, De amicitia, De divinatione, tr. William Armistead Falconer, Loeb, New York. 1938 [PA 6295.A3].
Tusculan disputations, tr. J.E. King. Loeb, New York. 1927 [PA 6304.T6 1971x].
The Letters to his Friends, tr. W. Glynn Williams, Loeb, Cambridge, MA. 1972-79 (4 vols.) [Widener Lc 37.519.86].
Rhetorica ad Herennium, tr. Harry Caplan. Loeb, Cambridge, MA. 1954 [PA6156.R4].
His "Dream of Scipio" was known to Chaucer with the commentary by Macrobius:
Ambrosius Aurelius Theodosius Macrobius, Commentary on the Dream of Scipio, tr. William Harris Stahl. New York. 1972 [PA6498.E6 S8].
For further references, see the Cicero Home Page maintained at the University of Texas at Austin. Many of his works, in the original and in translation, are available on the Internet Classics Archive maintained at MIT. (Unfortunately, the Archive does not have the Rhetorica ad Herennium, which is not available on the web).
For a biography of Cicero, see Plutarch's Life of Cicero, translated by John Dryden.