Reeve, C. D. C., Philosopher Kings: The Argument of Plato's Republic, Indianapolis: Hackett Publishing, 2006.

Excerpt:

Book I of the Republic differs markedly in philosophical style from its fellows. In it we find Socrates questioning all and sundry about what justice is, using the elenchus to refute them, and refusing to provide any positive answers of his own. This should make us uncomfortable. For we know that Plato later proscribes this practice altogether; dialectic should be employed only among mature adults, who have mastered the mathematical sciences. Why then does he begin the Republic with a lengthy example of Socrates violating this proscription?

Online:
Amazon