Plato's original texts. The texts
of Plato's various works that you'll find on the web are almost invariably taken
from William Jowett's famous 19th century translation of Plato's collected writings.
This is not surprising, for several reasons: Jowett's translations are in the
public domain, they contain certain quaint phrases that have become familiar to
English ears, such as "justice writ large," and they are generally quite
readable. However, I much prefer H. D. Rouse's edition, The Great Dialogues
of Plato, which is the one Palmer himself cites at the end of Chapter One
(p. 38). Rouse's editing decisions are more scholarly than Jowett's, and his translations
are more accurate and I think much more readable. You might also take a look at
Rouse's translations of the Iliad and Odyssey, which I like because
they don't force the Homeric epics into the format of English poetry (they look
and read like modern day novels, and not just because they are available in paperback).
Secondary works. The four secondary sources listed by Palmer at the end of his first chapter are all quite good, and I would only add that by some happy coincidnce they appear in exactly the same order in which I would recommend them. Thus I would suggest that you begin by looking at Cavalier's Plato for Beginners and work your way down the list to Vlastos. If you want to go on from there, drop by my office for more references.