According to editor David Gallop the Greek Parmenides (late 6th and first half of the 5th century BCE) was the first one who wrote a book in prose, and the first who can be considered a philosopher in the modern sense of the word. Only 150 lines of his sole work have survived, very fragmentarily, imbued with an obscurity that has since led to a library of interpretations.
Gallop here offers the original text of what has survived (or at least a reasoned reconstruction of it), a fairly literal translation, and above all, a great deal of explanation and commentary. In this explanation, he frequently takes a stand, with arguments of course, but Parmenides remains an inimitable author. It is perhaps no coincidence that Heidegger, in particular, made it his philosophical starting point.
I, for one, am surprised by the philosophical depth of Parmenides' work, as if he were the first to address the question of Being and Non-Being, and of the essence of Being in very abstract terms. That may be misleading, because similar musings can also be found in Egypt and Mesopotamian writings, but in much more veiled terms. Perhaps Parmenides was simply the first to do so, using an abstract terminology and a reasoning style that feel more familiar to us, now. And in that sense, therefore, it could be misleading. I am not well-versed enough to make an unequivocal judgment on this matter, but given the incomplete nature of his surviving work, I think it best to limit oneself to very cautious statements here.