The First Philosophers: The Presocratics and Sophists

There are few widely-available compendiums to choose from of the Pre-Socratics that include the Sophists, who are crucial for understanding Plato. Penguin has another that's not bad. This one is slightly better and more complete, hence if you want one and only one this is the way to go. Together they are complementary but in many ways redundant unless you want to compare translations. This book provides a plethora of the available fragments from all the important figures of the age, though it is not entirely exhaustive. Together with fine standard view introductions which ably assist the reader in navigating these complex and diverse materials, one effectively cannot go wrong in purchasing this useful, tidy, and cheap but sturdy little book (in this way its a good example of the Oxford World Classics series, and again, on this front they have the edge on Penguin, who seems to prefer to save a buck in printing costs). To get more of this material one must to go to the expensive dual-language Loeb series' and/or an Ancilla to the Pre-Socratic Philosophers, as well as many other secondaries. For a single, solid starting place, W.K.C. Guthrie's large, pricey, multi-volume history of Ancient Greek Philosophy is quite good and certainly the standard in handy reference works concerning this period - especially volumes I-III (III is mostly available now in the form of two books, simply called "Socrates" and "The Sophists".) His Plato books are fairly good, but mostly as starting points and reference guides to the dialogues, and the Aristotle volume is honestly not worth the money unless you can don't mind springing for a decent general, though not strictly light, intro or are consummately scouring secondary source material).