I'm afraid that I, unfortunately, found this book a bit of a disapointment. I greatly enjoyed Cartledge's book on Alexander the Great, but this book on Thermopylae was not his best.
I'm not an expert in this field but I have no doubt as to the quality of the scholarship, the facts and sources used, but there is really little on the battle of Thermopylae itself. i think there about six pages at best. However, the focus of the book appears to be the ancient, and modern, perceptions of the battle, and its influence on the ancient, and modern world. To be fair this is done well, as is the analysis of the Achaemenid Empire, Greece, and the Spartan state before the Thermopylae campaign. As a 'military' history I would say this book fails, but for quick understanding of ancient Greece, its mores, culture, religion and views of death and duty, this is a recommended read. The exploration of the Persian Empire under Darius and Xerxes, both culturally and militarily, is also worthwhile.
Cautiously recommended.