Oxford Classical Dictionary
Now handily available online, this is the major one-volume scholarly encyclopedia in the English language on all aspects of the Greco-Roman world. To learn not only what we know about a topic in antiquity, but how we know it, this is the place to look. The articles tell which ancient (and modern) authors tell us about an event, and exactly where in their writings to find it. The list of abbreviations for ancient authors and their works is the standard system used by classical scholars.