This book by Robert Graves is held by many as the definitive volume of Greek myths. Originally published as two volumes, it has since been consolidated. It covers everything when it comes to Greek myths. All of the major players (and most B-listers) are in this book.
Moreover, Graves includes alternate versions of many myths such as the origins of the universe, births of major deities, and their various (mis)deeds of mortals. While sometimes these alternate versions can be confusing (because which one is right?—they all are) these alternate tellings show how diverse and complex Greek myth is, and how this is not a body of work constructed by a single individual, but by an entire people.
Graves also includes translational and culture notes for every single myth fragment within the book, at the end of each myth in its own section. Graves even offers up cross-cultural influences from neighboring regions, and how the Greeks assimilated these influences to become natively Greek.
The book is organized chronologically according to the narrative timeline as opposed to the actual date of authorship. The book starts with the creation (obviously enough) and ends with the return of Odysseus. Obviously the Odyssey and the other epic poems contain a more detailed treatment of the Trojan War and the adventures of Odysseus, but this book isn’t a retread of Homer (and others), instead drawing from other sources of the myth.
I reach for this book first whenever it comes to anything related to Greek mythology. I can’t recommend it highly enough.