Or rather, to the many themes, because his work covers so much more than is in any other work. Some of the more important parts included are: The Theogony (Creation of the Cosmos and Gods), "Rape" (=Abduction) of Persephone, War of Gods and Giants, Prometheus' Fire, the Calydonian Boar, Sisyphus, Jason and the Argonauts, Medea, Bellerophon, Perseus, Hercules (all the great stories) and his children, Europa, Minos, Cadmus, Oedipus and Aftermath, Atlanta and the Apples, Aesculapius and Chiron, Helen's Early Years, the Palladium, Peleus, the Kings of Athens, Theseus, Tantalus, Atreus/Thyestes and all that Mess, Helen and the Trojan War, Achilles and the Iliad, the Odyssey and the other Returns from Troy.
So it's well that this is called The Library, because Apollodorus compresses a huge amount of information into four short books. So rather than being some of the dullest of ancient writing, as one reviewer says, it both treats the greatest stories and does so with economy and swiftness. This is not only a valuable reference book (as is Robert Graves's Greek Myths), but the work I often recommend as the best presentation for anyone who wants a no-nonsense overview of the whole of Greek mythology (and nice because it's one of the ancient Greeks themselves retelling the stories).
Now, if you want a cheap copy, just get the Oxford one. But if you want really excellent notes, get the Loeb edition annotated by Sir James Frazer (author of The Golden Bough), which also has some excellent short essays by him on themes in the stories.
And if you've been studying Greek, get the Loeb one too, which is literal enough to work as a good "pony", though the Greek is quite easy Alexandrian and you won't have any problems with it: My own Greek is not nearly as good as I'd like it to be, but I could read the whole thing in a few days no problem. The only thing I couldn't do is put it down!