I have been on a bit of a bender for reading ancient historians lately of the Roman Republic and so, Polybius is my latest reading in this area. I find reading ancient historians is perhaps the most effective way to both get some of the history and the mind set at the same time. Anyway it has worked and supplementing with Wikipedia where necessary, I finally feel like I have a basic understanding of the Roman Republic (I've known the Empire for a long time).
This work is selections from Polybius' Histories. Polybius was a Greek writing about Rome's rise to power over the Mediterranean from roughly the beginning of the first Punic war to the middle of the second century more or less. It was not entirely clear to me while reading this what was being not included because it has not come down to us and what was being excluded because it was considered less interesting and the work was being shortened. In effect this ends up being a history of Roman expansion from the beginning of the first Punic war to the end of the Second with some other stuff thrown in.
Polybius was an interesting person. He was a hostage following Rome's conquest of Greece and ended up falling in with a number of interesting people including a future King of Syria and some very powerful consuls. As a result he both traveled widely and knew what he was talking about. He also was a good researcher, who approached other Historians critically and went places, spoke to people and examined particular documents. That does not mean he is always neutral, in fact he often has an axe to grind especially when he is familiar at first or second hand with people who he can be too cruel or kind to. He is also not the most riveting writer.
However, he is a serious critical classical historian in the mode of Thucydides or Tacitus. He also has several interesting digressions on geography (he saw the landscape as dynamic and had personally been to many of the places involved. He also in interesting on the Roman military and its advantages and the Roman character both of which he admired. He furthermore, did quite serious theory of history and political theory. He advocated a mixed constitution, a constitution which he attributes to Rome (mostly incorrectly if you ask me). He was apparently quite influential on 18th century political theory and the American constitution is clearly indebted to him. He is also the first Greek of the Hellenistic period that I have read, which I found interesting in itself.
All in all he is an interesting to read to someone, who enjoys reading ancient historians. He is far from the best place to begin reading ancient history. I would begin almost anywhere else, but if you like this sort of thing, and I do, then well worth reading.