Diogenes Laertius (or, Laertios in the original Greek, instead of the customary, Latin, spelling) is the name of a compiler of biographies of philosophers; he is thought to have been active in the third century A.D., but possibly a little earlier or later (see the Wikipedia article on him for the range of possible dates, the possible sources of the name, and other details). The "Lives" shine rather narrow beams of light on thinkers who quite commonly took the whole universe as their topic, concentrating on what the compiler either thought his readers would want to know, or on the slender information he actually had. Even granting that he provides an authentic quotation, figuring out what a philosopher may have meant from Diogenes' context can be a challenge.
The author of "Lives of the Eminent Philosophers" is not to be confused with Diogenes the Cynic (a contemporary of Plato and Aristotle), best-known these days either for asking Alexander the Great not to block the sun, or for going about with a burning lamp in daylight, explaining that he was looking for an honest man. Or, for that matter, any other of the fairly numerous bearers of the name.
His ten-book compilation seems to be the survivor of a genre of digests of knowledge which were popular in late classical times, summarizing the heritage of the Greek past for the would-be-learned. The information he offers ranges from the demonstrably inaccurate to the apparently trustworthy; his final book, on Epicurus, is generally regarded as his best, containing as it does substantive quotations from treatises and letters, which pass language and style tests for authenticity. It has often been suggested that he was himself an Epicurean, or as close to one as the eclecticism of the philosophical schools of his time (whenever exactly it was) allowed. (Typically, he has also been classed as a Skeptic....)
Annoyingly, some of the most important, and most disputed, information for which he is the sole or chief source comes in the earlier parts of the "Lives," in the portions concerning the so-called "Pre-Socratic" philosophers. Friedrich Nietzsche (the philosopher), who published a pioneering study of Diogenes Laertius early in his short, brilliant, career as a classical philologist, suggested calling them "Pre-Platonic," since their works, along with those of Socrates, survive only in quotations, whereas Plato was the first to leave a substantial body of surviving work. Unfortunately (as I have complained in another review), the less appropriate name is the one that has stuck -- even though some of the "Pre-Socratics" were his younger contemporaries.
Diogenes here seems to be working with previous compilations and summaries, and may have known little more about their works than the brief passages he actually quotes -- as opposed to Simplicius, a commentator on Aristotle who provided extended quotations from some of the earlier philosophers Aristotle mentioned, noting that their writings had become difficult to find. Inevitably, there is a substantial critical literature, in which Diogenes' contributions are sliced thin in order to be offered as evidence, or rejected as unreliable. The information in Diogenes is scrutinized and dissected in a standard (but increasingly recognized as obsolete) edition compiled in the nineteenth century and several times revised, Diels' "Fragmente der Vorsokratiker" (now referred to, from an editor of a revised edition, as Diels/Kranz). I have discussed several English versions in a review of Kathleen Freeman's "Ancilla to Pre-Socratic Philosophers: A Complete Translation of the Fragments in Diels, 'Fragmente der Vorsokratiker'." (See below some of the other titles.)
The immediate subject of this review is "Diogenes Laertius: The Lives and Opinons of Eminent Philosophers," a Kindle edition based on the 1853 translation by C.D. Yonge, originally published in Bohn's Classical Library, which was a sort of Victorian precursor to Everyman's Library, the Penguin Classics, and the Oxford World's Classics (and various series by other publishers). The translation is regarded as fairly literal, and complete, but not without errors. Yonge also smooths over (but does not omit) material offensive to Victorian -- and more modern -- moral sensibilities. I find it quite readable, but others may balk at the mixture of Victorian English and the underlying Greek sentence structures. As with any translation of Diogenes Laertius, it must be read with critical attention, and not taken too seriously as evidence without further examination.
The bilingual Loeb Classical Library edition by R.D. Hicks ("Lives of the Eminent Philosophers," 1925; two volumes) is generally regarded as superior, but, as noted in the Wikipedia article on "Lives and Opinions," his translation is bowdlerized. (Some of the volumes in the earlier decades of the Loeb Classical Library translated such passages in Greek into Latin, and those in Latin into Italian, providing the passages with "the decent obscurity of a learned language," with sometimes amusing results.) There is a on-line version of Hicks' translation from Wiki-Source, and PDFs from the Library of Congress's archive.org website, and print versions are usually available from Amazon.
A third translation, slightly abridged if memory serves, is the paperback "Lives of the Philosophers" (1969), translated and edited by A. Robert Caponigri. I read it sometime in the 1970s, and no longer have a copy to refer to. My impression was mostly favorable, but, as I recall, Caponigri left out the traditional book and chapter numbers, which made finding references a challenge. The lack of commentary or bibliographic references was also a disappointment.
A (rather expensive) critical edition has recently (2013) been published, "Lives of Eminent Philosophers," edited by Tiziano Dorandi, as volume 50 in the Cambridge Classical Texts and Commentaries series. With luck, we may see a new translation (or two) based on this edition sometime in the future -- Penguin Classics and Oxford World's Classics being, I would think, likely publishers. Of course, bits and pieces appear in a Penguin Classics volume edited by Jonathan Barnes , "Early Greek Philosophy" (1987, second edition 2002), and in Robin Waterfield's "The First Philosophers: The Presocratics and The Sophists" for Oxford World's Classics (2000).