Plato's Theory of Forms has been the subject of commentary for thousands of years, and many new assessments of the theory appear every decade. But the starting point for all discussions of the theory is with Plato himself. Plato lays out the theory, with variations, in the Republic (especially from the end of book five to the end of book seven), in the Symposium, in the Phaedo, and in the Parmenides. And there are other remarks on the theory scattered throughout Plato's work. Michael Shenefelt discusses aspects of the Theory of Forms in a chapter called "Is Morality Objective?" in his book The Questions of Moral Philosophy, published by Prometheus.
Plato's dialogues supply the most extensive contemporary portrait of Socrates that we have--though many commentators believe that Plato also uses the character of Socrates from time to time as a mouthpiece for his own theories. Plato's early dialogues are often thought to be the closest that we shall come to the historical man behind the legend. Plato's Apology gives an account of Socrates' trial. Plato's Euthyphro offers an excellent example of Socratic method at work, and so does Plato's Meno--especially when Socrates is shown proving a special case of the Pythagorean theorem merely by asking questions (82b-85c). Plato's Crito shows Socrates refusing to escape from prison--yet maintaining his innocence. And Plato's Phaedo shows Socrates insisting on the immortality of the soul just before, in obedience to a court order, he drinks the hemlock that ends his life.
