Theogeny, Works and Days
by Hesiod
Hesiod was an ancient Greek poet, generally thought to have been active around the same time as Homer. Theogony is commonly considered Hesiod's earliest work and tells the origin story of the Greeks' world and gods. In terms of the timeline, pretty much everything we know about Greek mythology begins with Hesiod. The Works and Days is a poem (around 800 lines) in which Hesiod instructs his brother in the agricultural arts. It is based around two general truths: labour is the universal lot of Man, but he who is willing to work will get by.
Stanley Lombardo's 1 translation by Hackett publishing of both works is highly readable and entertaining, and comes with a set of notes for each. The 2 Penguin Classics version translated by Dorothea Wender is also quite readable although a looser translation of the original Greek.