When the Greek city-states turned to this new hoplite way of fighting, it meant that each man had to have the right armor. Nobody could fight without a shield, and a helmet, and all that. So you could only be a soldier if you could afford the hoplite armor. The armor was pretty expensive, and the expense kept a lot of poor men from being in the army. But it made each of the men who were still in the army very important – one weak link, and the whole chain would break and the battle would be lost.
Some people think that this new emphasis on the importance of each ordinary soldier (instead of just the rich, powerful heroes of the Iliad) helped democracy to develop in Greece. These men knew the city-state needed them to fight, so they started to demand a vote in what wars they were going to fight in.
In addition, the hoplite phalanx made Greek soldiers very good fighters, so that richer countries, like Lydia, the Persian Empire, and Egypt, wanted to hire them as mercenaries for the next several hundred years. Those well-paid soldiers brought a lot of money back to Greece. These same hoplite tactics helped the Greeks to fight off those same Persians during the Persian Wars.
Greek Hoplite (Soldier Through the Ages), by Martin Windrow (1985). , from Scholastic.
Greek Hoplite 480-323 BC, by Nicholas Sekunda (2000). From Britain. A good first guide, useful for painting models or illustrating reports.
Warfare in the Classical World: An Illustrated Encyclopedia of Weapons, Warriors and Warfare in the Ancient Civilisations of Greece and Rome, by John Warry. Lots of pictures, and not too much text.
Greece and Rome at War, by Peter Connolly (1998). Not too hard.
Hoplites: The Classical Greek Battle Experience, edited by Victor Davis Hanson (reprinted 1993). Essays by different specialists, more or less accessible to interested and confident readers. The writers don’t discuss strategy and tactics so much as the experience of the actual individual soldier.