This book is kind of outdated in its information and definitely so in many of its attitudes, but nonetheless it remains a bit of a classic. I think that’s mostly because of the author’s sheer enthusiasm for the people about whom he writes, their land and their customs. I studied Athenian democracy in excruciating detail for a Classics A Level, but Kitto manages to actually get excited about it, to show all the best things about it and the way the Greeks behaved and thought. It’s mostly about the Athenians, honestly; you can consider the two basically synonymous in this book — Kitto does talk about the Spartans, for instance, but with significantly less approval and interest.
Kitto’s style is mostly engaging due to his enthusiasm, but I do warn that he quotes extensively from various sources (rather than summarising them, he lets them stand for themselves to illustrate his points; this can get tiresome).
Just as a warning, though, if you were thinking of picking this up: though I do think there’s something charming about Kitto’s complete adoration of the Athenian people, he definitely held some less than charming opinions about the place of women and the treatment of slaves — he thought that most things were justified because it allowed the Athenians to have their genuine democracy (which just so happened to exclude much of the population).