Benjamin Jowett's complete five-volume translation of Plato's dialogues -- the primary source for nearly everything known of Socrates's actual teaching, since Socrates himself wrote nothing.
Plato's dialogues are the single most important body of philosophical writing to survive from antiquity, and they present a genuine puzzle: nearly all of them feature Socrates as the central speaker, but Socrates himself left no writing at all -- everything we know of his actual teaching comes filtered through Plato (and, separately, through Xenophon, elsewhere in this Oedio). Scholars still debate how much of "Socrates" in these dialogues is the historical man and how much is Plato using him as a mouthpiece for his own evolving thought, especially across four decades of writing that moved from tightly focused ethical questions (What is courage? What is piety?) to the vast metaphysical and political architecture of the Republic and Laws. Benjamin Jowett's translation, first published in 1871 and revised for this 1875-1892 second edition, was for generations the standard English Plato -- comprehensive, stately, and still eminently readable.