I recently had a long layoff in finishing any books because I was slowly making my way through Thucydides' "The History of the Peloponnesian War."
His history is rather dense, but quite remarkable for something written in the 5th century B.C.
As for the war itself, it was a long -- about 27 years -- conflict between Greek powers Athens and Sparta and numerous allies on both sides. It started for many reasons, but most notably because of Athens' expansion of its Aegean empire and its friction with Sparta, which felt threatened. The eventual defeat of Athens reshaped Greece.
Unlike an earlier historian, Herodotus (who I wrote about here), Thucydides attempts a chronological, factual account of the war. The author served as a general in the Athenian army for some of the time and seemed to have good sources otherwise, though he fails to name them.
Thucydides appears -- though this has been disputed -- to remain objective in his account and to shun propaganda. He states early on that, "My work is not a piece of writing designed to meet the taste of an immediate public, but was done to last forever."