Apollodorus : The Library I, Books I-III.IX

Basically, I woke up one day and decided that I wanted to read about Greek mythology. I figured I would give these volumes a try, since they seem to be a no-nonsense re-telling of the whole of Greek mythology.

According this edition's translator, J. G. Frazer, this text was originally attributed to Apollodorus of Athens, who was born around 180 B.C., but the text was actually written by someone else during the first or second century A.D. This edition is heavily annotated, and you get the original Greek text and English translation side-by-side. Even if, like me, you can't read ancient Greek, it still looks cool, and makes you look smart when people see you reading it.

You want to know where all the ancient Greek gods are described? How about the Heroes? They are all here in this two volume set. Appolodorus compiled the Greek myths here. Undoubtedly used by all subsequent (post 140 AD) compilers of ancient myth, this is the source. Although Hesiod and Pindar (and possibly Ovid) were his sources, you would have to read each of them to find but a quarter of the material on mythology.