Classical References to the Celts
Due to their close relationship (mostly antagonistic), it is only natural that the Greeks and Romans would write about their neighbors. These early writings on the various Celtic tribes, their culture and religion are quite illuminating, as the Celts left no writings of their own until several centuries after the introduction of Christianity. Some, like Tacitus, are somewhat sympathetic to their subject, while others, such as Pliny, regard them as barbarians with con-men for leaders. C'est la vive.
Greek Texts:
Aristotle (384 BC – March 7, 322 BC)
Eudemian Ethics, Nicomachean Ethics, and Politics.
Diodorus Siculus (fl. 50 BCE)
Library
Diogenes Laertius (fl. 230 CE)
The Lives and Opinions of Eminent Philosophers introduction
Herodotus (~440 BCE)
The Histories
Plato (427-347 BCE)
Laws: On Drunkeness.
Polybius (~203 BC - 120 BC)
The Histories: off site, as much of the book talks about the Celts.
Ptolemy
Geography: off site.
Strabo (~64/63 BCE - 24 CE)
Geography: Book Three: Iberia
Geography: Book Four: Celtica
Latin Texts:
Julius Caesar (July 13, 100 BC – March 15, 44 BC)
The Gallic Wars: selections from book six
Clement of Alexandria
The Stromata
Hippolytus, St.
The Refution of All Heresies
Jerome, St.
Celtic Cannibalism: from Against Jovinanus.
Marcus Junianus Justinus (3rd C. CE)
Epitome of the Philippic History of Pompeius Trogus: on the Gallic sack of Delphi.
Aelius Lampridius
The Life of Severus Alexander
Livy (~59 BCE - AD CE)
The History of Rome: Book Five: The Gauls destroy Rome.
Lucan (November 3, AD 39 - April 30, 65)
Pharsalia: selections from book one
Origen
Against Celus
Pliny (23 - 79 CE)
Natural History: selections from various books
Suetonius (75-160 CE)
Selections from The Lives of the Twelve Caesars
Julius Caesar
Augustus
Caligula and Claudius
Nero
Tacitus (c. 55–c. 117 CE)
The Life of Agricola
Annals: selections
History: selections from book four
Tertullian
De Anima
Vopiscus
Diocletian and the Druidess: from the Historia Augusta.