Caesar's Commentaries on the Gallic War are his account of his campaigns of 58-50 BC. The apparently objective, third person style of the commentaries probably conceals a propaganda purpose - shoring up Caesar's position in the political struggles at Rome which would lead to the conflict with Pompey and the Senate recorded in his later Commentaries on the Civil War.
The Commentaries are notably as a description of Celtic Gaul, and an early account of much of Western Europe. Caesar's campaigns took him across the Rhine into Germany and across the channel into Britain, and often involved diplomatic alliance-building as much as fighting, at least until the general revolt of the Gauls under Vercingetorix which ended with the climactic siege of Alesia. An eighth book, by Caesar's lieutenant, the later consul Aulus Hirtius, covers the period between Alesia and the Civil War.
The Gallic War is often the first authentic text used in introducing students to Latin, much as Xenophon's Anabasis is for Ancient Greek. Some of the free resources below may be useful for that purpose notably the bilingual Loeb and Perseus texts, and the Memrise flashcard app. Dr Butler's 1851 map is still useful for following the narrative.
The Gallic War at online book stores
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Free online texts
Gutenberg: "De Bello Gallico" and Other Commentaries by Julius Caesar, translated by Thomas De Quincey. Multiple formats.
Gutenberg: C. Iuli Caesaris De Bello Gallico by Julius Caesar. Latin text. Multiple formats.
Internet Classics Archive: The Gallic Wars, by Julius Caesar, translated by W. A. McDevitte and W. S. Bohn. HTML and text formats.
Latin Library: C. IVLI CAESARIS COMMENTARIORVM DE BELLO GALLICO LIBER PRIMVS. Latin text. HTML format.
Loebulus. L072 - Caesar -- The Gallic War. PDF of public domain Loeb edition in Latin and English.
Standard Ebooks: Commentaries on the Gallic War, translated by W. A. McDevitte and W. S. Bohn. AZW3, EPUB, KEPUB and Advanced EPUB formats.
Perseus: Latin text (T Rice Holmes ed., 1914). English translation (McDevitte & Bohn, 1869). HTML texts.
Wikisource: Commentaries on the Gallic War, Online text.
BBC Radio Great Lives: Julius Caesar. Matthew Parris with Barry Cunliffe.
BBC Radio 4 In Our Time: Julius Caesar. Melvyn Bragg with Christopher Pelling, Catherine Steel and Maria Wyke.
Dickinson College Commentaries: Selections from the Gallic War - Latin text, notes, vocabulary, and media for selections from The Gallic War by Julius Caesar, intended for readers of Latin.
Gallia: Map from Dr. Butler's Atlas of Ancient Geography by Samuel Butler, London: Longman, Brown, Green, and Longmans, 1851.Perry-Castañeda Library
Librivox: Commentaries on the Gallic War - public domain audiobook.
Memrise: AP Latin Caesar, by billclausen. One of a number of relevant vocabulary courses on the site.
Wikipedia: Commentarii de Bello Gallico