Any book on political philosophy is, in general, useful for political theory. It's hard to say what the difference is between theory and philosophy. G A Cohen argues that the difference is, if there is a difference, that political theorists are interested in what to _do_, sometimes even what to do _here and now_. Political philosophers are not. Instead they deal in supposedly eternal questions, like 'what is justice'? This reflects Cohen's work, certainly, but I'm not sure I agree with the dichotomy. Either way, Miller was always more of a theorist than a philosopher. Especially according to Cohen's definition..