(Here's a follow-up comment that I should probably lodge here, so it doesn't get completely lost:
This book is a disgrace, while Kirk was a competent scholar, his book on Heraclitus is quite good, though flawed, Raven was an idiot... and Schofield has here made an old book that at least was servicable almost worthless. They are completely unreliable guides to what the Presocratics, both individually and collectively are all about. Sorry to burst anyone's bubble....
Here's Kirk's book on Heraclitus:
Here's an example of the type of thing that J.E. Raven published
In general, the Kirk half of K&R had more value than the Raven half -- but old' Schofield ruined both halves.
On the Presocratics, one simply has to study Cherniss' book, Aristotle's Criticism of Presocratic Philosophy (which is quite difficult), and then study the relevant papers of William Arthur Heidel (Heidel, Selected Papers). And then read the specialized monographs on individual figures. Burnet's book, Early Greek Philosophy, is absolutely brilliant, but is completely wrong on almost everything - but it is the sort of brilliance that makes it a book that is worth not just reading, but studying. Guthrie's two volumes (I & II) in his History of Greek Philosophy that are dedicated to the Presocratics are useful in that they summarize much bibliography and often states the status questionis of a problem in clear terms (and they are very readable) - but Guthrie was himself a fool and missed a lot of what, after Cherniss and Heidel, should have been obvious.
One fabulous monograph that very few will have heard of is J.W. Beardslee's Physis (which was a dissertation done under the supervision of Paul Shorey):
On Parmenides, you simply have to read and study and understand Tarán's book -- but that is very difficult if you don't have access to the Greek. So, if you're interested in Parmenides and don't know Greek, try Kant. Or somebody like that....