Hey, the best source on this Period is Herodotus the histories, for Sparta's contribution I would look in book seven as it has the battle of Thermopylae which is a signifigant factor in Sparta's war contribution. Sparta set an example in this battle for the rest of Greece, to win or die trying which boosted Greek morale. Also, Sparta held off the Persian advance by up to seven days which gave Athens time to evacuate (refer to the troezen inscription). Hope thats helpful =)