If you're new to Classical Mythology, you should start with something like Edith Hamilton's Mythology, Robert Graves' The Greek Myths, or Richard Martin's "Myths of Ancient Greece" as an introduction. Then you probably want to read Homer, Hesiod, and the Greek Tragedians in full.
Once you've read through those, as well as probably Ovid, the Homeric Hymns, and Vergil, this is where you want to go next. The editors have assembled an impressive collection of translated primary sources. Lesser known writers included in this volume include Aelian, Antonius Liberalis, Apollodorus, Babrius, Conon, Diodorus, Eratosthenes, Hellanicus, Heraclitus, Herodotus the mythographer, Hyginus, Lucian, Lucretius, Palaephatus, Parthenius, Pausanias, myths from Plato's dialogues, Proclus, Xenophanes, and others. This is an amazing resource. The only other way you'll get this wide a variety of classical sources is if you buy around 20-30 separate Loebs.
This edition is a steal. The introductions for each writer as well as the extensive appendices which contain full translations of various Near Eastern sources are all useful.