Diels, H. Die Fragmente der Vorsokratiker. Berlin: Weidmannsohe Buchhandlung.
This collection of Presocratic fragments by Hermann Diels is one of the most important works of classical philology. First published in 1903 it was later revised and expanded three times by Diels, and finally revised in a 5th edition (1934–7) by Walther Kranz and again in a sixth edition (1952). It gives for each of the Presocratic philosophers quotations from their mostly lost works by later writers and secondary-source material known as testimonia. Various editions are accessible to download free online: Die Fragmente der Vorsokratiker, griechisch und deutsch
For an English translation of large selection of the fragments, with an erudite commentary and the Diels numbering for easy cross referencing, see: Kirk, G.S., Raven, J.E. and Schofield, M. (1984). The Presocratic Philosophers. A Critical History with a Selection of Texts. Cambridge: Cambridge University Press.
Parmenides
The Fragments of Parmenides. A critical text with introduction, translation, the ancient testimonia and a commentary by A. H. Coxon (Assen/Maastricht: Van Gorcum, 1986).
Burnet, J. (1971). Early Greek Philosophy. London: A. and C. Black.
Burnet’s philological work on Plato is still widely read, and his editions have been considered authoritative for 100 years, as the 5-volume Oxford Classical Texts critical edition of Plato works and spuria (1900–1907). His commentaries on Plato’s Euthyphro, Apology, and Crito and on the Phaedo also remain widely used and respected by scholars. Four editions of Early Greek Philosophy were published by A. & C. Black, Ltd. in Great Britain: the first edition in April 1892, the second in June 1908, the third in September 1920 and the fourth, posthumously, in 1930. The second edition is available online: Early Greek Philosophy, by John Burnet
Dudley, D. R. (1937). A History of Cynicism. London: Methuen & Co.
Jaeger, W. (1947). The Theology of the Early Greek Philosophers. Oxford: Clarendon Press.