Polybius is a Greek writing to Greeks trying to explain how Rome came to dominate the Mediterranean. His 6th book is an analysis of Rome's political and military practices systematically including the structure of the legion, recruitment, provisioning, layout of the camp etc.

The rest of it is a military history starting with the 1st Punic War down to 146, the year in which Rome destroyed Carthage and Corinth.

Polybius lived for years in Rome as a hostage in the house of Paulus and apparently traveled widely in Italy and was friends with Scipio Aemelianus, so he probably has a fairly good idea what he's talking about.

LacusCurtius has the Loeb translation up.

In Plutarch's Life of Crassus, the author likely uses a first hand Mesopotamian source to describe the battle of Carrhae. This is because his account goes into great detail about the battle and aftermath in Ktesiphon.