Some background: the Peloponnesian War was fought between Athens and its "allies," who were more or less client states, and Sparta and its allies, who had considerably more independence. The war ended, after almost 30 years of fighting, with the defeat of Athens. ThePeloponnesianWar is a more or less straightforward accounting of the war, mostly in chronological order. Like many books written before the modern era, Thucydides never finished it; he essentially spent his entire life working on it, and he died before he got to the end of the war. Thucydides had peculiar biases for a historian of his time: he discusses very early the lack of reliability of eye-witness accounts and the necessity to find multiple sources to construct a true picture of an event. Apparently, Thucydides went to great lengths to interview men on both sides of each of the various conflicts, and to verify that what he recorded was actual truth. On the other hand, Thucydides never documented his sources, and while his book is replete with speeches supposedly delivered by various important figures in the war, there are very good reasons to believe that they bear little resemblance to what was actually said on these occasions. In other words, while I think that Thucydides had very modernistic inclinations toward historical accuracy, he simultaneously viewed things through the perspective of the ancient Greeks in which his various violations of current historical custom were not just permitted, but expected.
The war itself has a very different flavor from practically any other I am familiar with. As was noted in my discussion of TheFaceOfBattle, Thucydides describes his battles in a very different manner than that of most modern chroniclers and that most famous other writer, Julius Caesar. He places great emphasis on the psychology of the men involved. The difference from modern battles are also emphasized by the relatively small numbers of troops involved. In post-Renaissance Europe, armies were composed of tens of thousands of troops at least and one hundred thousand was not uncommon; as time passed, the armies only got larger. In the Peloponnesian War, an army of ten thousand was large, and most significant fighting forces had only several thousand men. This influenced the war in countless ways, none of which I can point to specifically (if you want to see, read the book). Thucydides' descriptions of sieges also illuminated the ingenuity and desperation that characterized siege warfare better than any other source I have seen.
Thucydides' style has been described as uncompromising, and I have to agree. The book is rather long and not very easy to read; it took me about 3 weeks to finish, reading it off and on. I've heard that ProfessorDavis uses this as a text in his Politics and Power I class; while he does have traditionally high expectations for the amount of reading that his students should do, I don't see how it would be possible to finish the entire book during the semester along with reading anything else at the same time.