A thorough, yet concise history by a foremost scholar
Donald Kagan, one of the foremost scholars of Ancient Greek history, wrote a concise but thorough history of the Peloponnesian War for a general audience based off his four-volume academic masterpiece on the same subject. From the start Kagan brings the reader to the time period of the war with enough background information that someone not familiar at all with Ancient Greece will understand the circumstances of the beginning of the war from each side's viewpoint. Throughout the work, Kagan brings in a modern military and political view to help examine decisions of either side that the ancient sources' explain as social virtue or vice. This supplement to the ancient sources helps give a fuller view of the decisions of the Athenians, Spartans, and their respective allies. If you want to learn about Ancient Greek history beyond Marathon or Thermopylae, I fully recommend this book.