As I wrote earlier, Livy's history is by far the most detailed account of Rome, which means in most cases there is no "elsewhere".
It's through Livy that we know about the churn of the republican political system: the periods of rule by the decemvirs, the military tribunes with consular powers, and the consuls. No other author gives us more information about the contention between the plebeians and the patricians, and the rise of the plebeians as they were admitted to more and more magestries.
Livy mentions the establishment of almost all coloniae established in the time periods covered by his surviving books, and in many cases tells us how many colonists participated in its foundation. He informs us when new tribes were created, indicating the enfranchisement of new citizens. He tells us far more than any other author about the responsibilities of the various magestries and the operation and scope of the Roman assemblies. It's through Livy's mention of what man held what office that we are able to piece together which of the lesser families were plebeian and which were patrician. The great majority of what we know about Roman laws prior to the mid 2nd C BC is known through Livy. Livy tells us who the consuls were for virtually every year in which consuls were elected, and he is our only source for most years for which the fasti are illegible. Particularly in the later surviving books, Livy often goes into great detail about the number of Roman and allied soldiers raised, and how many formed new legions and how many were used as replacements.
Livy is indispensible: if one were to read only one of the ancient historians of Rome it would be him.
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