The Peloponnesian War, along with the myriad feuds that latched on to the central conflict between Sparta and Athens in the latter half of the fifth century BCE, can be an exhausting subject. The civil and international politics involved in fostering and perpetuating the war rival even today's most complex conflicts.
In this, the third of four volumes on the subject, Kagan skillfully presents, comments on, and refutes the hypotheses presented by history and historians, while still managing a very approachable narrative. These books come in and out of circulation, so best to get ahold of them while they're available. Again, Kagan's work is superb
For the historian, or avid history buff (however you might self-identify), these works are a necessary addition to your library. The more casual reader might, however, consider purchasing Kagan's abridged work entitled simply "The Peloponnesian War." It includes the main thrust of the narrative, but with markedly less analysis of the political motivations included in these volumes.