Specifically on the Peloponnesian War, Thucydides' History of the Peloponnesian War is our main primary source, so it's a great place to start if you want to go straight to the heart of what we know. However, if you're looking for a more general introduction to the period first, I would recommend R. Osborne's nicely compact Classical Greece, 500-323 BC (2000), which has individual chapters on 5th-century history and on Greek warfare by leading scholars in the field, or his solo introductory textbook, Greek History (2004). You also can't go wrong with A. Powell's Athens and Sparta (2nd ed. 2001).

In terms of the Peloponnesian War itself, the book I recommend these days is J.E. Lendon's phenomenally engaging Song of Wrath: The Peloponnesian War Begins (2010). It's not expensive either. If you're interested in Greek warfare as such, go for H. van Wees' Greek Warfare: Myths and Realities (2004) or L. Rawlings' The Ancient Greeks at War (2007).

Most places you look will recommend D. Kagan's The Peloponnesian War (2004), which is a summary of his earlier multi-volume series on the topic, and is pretty universally available. Personally, I wouldn't recommend this work; it mostly just rehearses what Thucydides and Xenophon say, so you can just go straight to the primary sources and get largely the same information. Also, Kagan's work is typical of a retired emeritus professor - a retread of the research he once did, but which is now several decades out of date.

Whatever you do, do not read V.D. Hanson's A War Like No Other (2005). It is worthless.

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