The people of Gaul (modern France) didn't know what they were getting into when they asked Rome for help. Some of the Gallic tribes were official Roman allies, so Caesar was obligated to come to their assistance when they asked for help against the incursions of stronger, Germanic tribes from across the Rhine. Not one for modesty, Julius Caesar wrote about his campaigns in Gaul in third-person accounts that are known as Caesar's Gallic Wars (or in the singular, as Caesar's Gallic War, or as "Commentaries").
Julius Caesar wrote commentaries on the wars he fought in Gaul between 58 and 52 B.C., in seven books, one for each year. This series of annual war commentaries is referred to by various names, but is commonly called De bello Gallico in Latin, or The Gallic Wars in English. There is also an 8th book.
Read Caesar's Gallic Wars online in a public domain translation from 1915. Be sure to read Thomas De Quincey's Introduction to the Gallic Wars for historical background.
Adrian Goldsworthy's Caesar - Life of a Colossus is a long, thorough, readable biography of Julius Caesar written by a military historian who includes great detail on the times and customs of the late Republic. If you aren't terribly familiar with Julius Caesar, Goldsworthy provides you with the events in his fascinating life