Possible experience Book

Francis Cornford does a decent job of presenting these dialogues; and, as the name indicates, these dialogues are dedicated to Plato's philosophy regarding knowledge, and elucidates his theory of forms. I have read Cornford's translation and commentary on Plato's Timaeus and thought Cornford had a very good understanding of the text; although, I didn't always agree with his conclusions. Like that commentary, I don't necessarily agree with all of Cornford's conclusions here either; but, over all, I think he has a pretty good grasp of the text.
There are some other dialogues that round out the information Plato presents here. Along with the Theaetetus and the Sophist, the dialogues: Philebus, Timaeus, Parmenides, Meno, the Republic and to a degree, Phaedrus, Phaedo and the Symposium can be included and do present further important aspects of Plato's philosophy of forms and/or knowledge. I do like the fact that Cornford regularly supplies the Greek text in footnotes and in the commentary; so one knows when Plato is using terms like ousias, eidolas, ontos, genos etc. That is a complaint I have with many translations where the subject matter can often be ambiguous. It is great to have the underlying text so one can gain a little bit better comprehension.
In the Theaetetus, Plato refutes Protagoras' relativistic philosophy (i.e. that man is the measure); and in the Sophist he largely refutes Parmenides' ideas regarding absolute unity and non-being equaling non-existence. I did find Plato's insistence that non-being equates simply to difference, not to non-existence, to be incredibly interesting. You find similar notions later on in idealists like Schelling. Plato also breaks down existence into distinct categories -not in the Aristotelian sense, however. Categories have regard to "kind", "sameness", "difference" etc. Plato is seeking to find the underlying form as a means of determining the source of division in these categories. His main objective seems to be to deduce what forms are basic and can either be joined to, or are absolutely distinct from and opposed to, other forms. He also investigates perception in these dialogues and refutes it's reliability as a source of knowledge.
As it stands, these are essential dialogues in the Platonic canon, and as such, well worth reading.