Titus Livius (around 59 BC - AD 17), known as Livy in English, wrote a monumental history of Rome, Ab Urbe Condita, from its founding (traditionally dated to 753 BC) through the reign of Augustus. Livy was a native of Patavium (modern Padua, Italy) in Cisalpine Gaul.
The book's title, Ab Urbe Condita ("From the Founding of the City"), expresses the scope and magnitude of Livy's undertaking. He wrote in a mixture of annual chronology and narrative—often having to interrupt a story to announce the elections of new consuls as this was the way that the Romans kept track of the years. Livy claims that lack of historical data prior to the sacking of Rome in 386 BC by the Gauls made his task more difficult.
Livy wrote the majority of his works during the reign of Augustus. However, he is often identified with an attachment to the Roman Republic and a desire for its restoration. Since the later books discussing the end of the Republic and the rise of Augustus did not survive, this is a moot point. Certainly Livy questioned some of the values of the new regime but it is likely that his position was more complex than a simple 'republic/empire' preference. Augustus does not seem to have held these views against Livy, and entrusted his great-nephew, the future emperor Claudius, to his tutelage. His effect on Claudius was apparent during the latter's reign, as the emperor's oratory closely adheres to Livy's account of Roman history.
Livy's work was originally composed of 142 books, of which only 35 are extant; these are 1-10, and 21-45 (with major lacunae in 40-45). A fragmentary palimpsest of the 91st book was discovered in the Vatican Library in 1772, containing about a thousand words, and several papyrus fragments of previously unknown material, much smaller, have been found in Egypt since 1900, most recently about forty words from book 11, unearthed in the 1980's. Livy was abridged, in antiquity, to an epitome, which survives for Book I, but was itself abridged into the so-called Periochae, which is simply a list of contents, but which survives. An epitome of books 37-40 and 48-55 was also uncovered at Oxyrhynchus. So we have some idea of the topics Livy covered in the lost books, if often not what he said about them.
A number of Roman authors used Livy, including Aurelius Victor, Cassiodorus, Eutropius, Festus, Florus, Granius Licinianus and Orosius. Julius Obsequens used Livy, or a source with access to Livy, to compose his De Prodigiis, an account of supernatural events in Rome, from the consulship of Scipio and Laelius to that of Paulus Fabius and Quintus Aelius.
A digression in book 9, sections 17-19 suggests that the Romans would have beaten Alexander the Great if he lived longer and turned west to attack the Romans, making this the oldest known alternate history.
Many of Livy's comments on Roman politics seem surprisingly modern today. For example, he wrote (of the year 445 BC):