Polybius (ca. 200–118 BC), Greek Πολύβιος) was a Greek historian of the Hellenistic Period noted for his book called The Histories covering in detail the period of 220–146 BC. He is also renowned for his ideas of political balance in government, which were later used in Montesquieu's The Spirit of the Laws and in the drafting of the United States Constitution.
It is easy to see why Polybius has had an impact throughout the ages. He interrogates his subjects and sources and does not accept his own country's myths or stories as gospel. I can't add much more to what's been said about him and his impact, but I did increase my knowledge of the Roman Empire, Hannibal's march, and the origins of Rome's conflicts with Carthage.
The book is well annotated, the maps are useful, and easy to use. There is a minor flaw in this printing: the text frequently changes size (it is either scrunched up or elongated). However, it's still a fascinating book, and I'm glad to have experienced its wisdom.