If you are wanting books about the ancient Greeks, a truly great one is Before and After Socrates by F.M. Cornford. The book is very short, and very expensive for how short it is. But it is worth the purchase price for the chapters on the Presocratics and on Socrates. He is concise and clear and very informative. This may well be the best introduction to the Presocratic philosophers ever written. Cornford was a great scholar.
"A wise man ... proportions his belief to the evidence."
— David Hume, An Enquiry Concerning Human Understanding, Section X, Part I.