I first read Kirk and Raven's `Presocratic Philosophers' as an undergrad. I was being taught pre-socratic philosophy by a Cambridge Classical Greek philosopher. Since then it has become one of my favourite and most valuable philosophy texts. Kirk and Raven present us with all the credible `fragments' of presocratic philosophy as quoted in later philosophers. They present the original greek and a (most times) definitive english translation and analysis. Although, what I learned through my professor via Kirk and Raven is that you sift or decant yourself through these most ancient and profound fragments - more akin to an approach to the `I Ching' and poetry than contemporary analytic philosophy. Kirk and Raven present us with the `holy' text of western philosophy which never fails to produce the wonder of thinking - a true thinking that is rare and primordial. Even if you usually don't like reading philosophy, the presocratics are really `post-modern' and poetic in their fragmentary and oracular collages of meaning. T.S. Eliot's `The Four Quartets' is soaked with pre-socratic philosophy.
The Pre-socratics deserve more attention because `all', and I mean all, of the basic philosophic and scientific positions are contained within these seeds of the western tradition.
This is the standard english language academic text book on the Presocratic Philosophers in Ancient Greece. It broke hundreds of years of tradition by including Orpheus, Musaeus, Pherecydes etc. - 'philosophers' before the Ionian Physicists.