I discovered the two volume hardcover set in the local library and thus began my years of checking them out and lugging them home until my husband bought me the set for Christmas. Might be the most important books I own. As a writer of the mythic I often turn to this source. If I don't know what I feel like writing next, I just spend a night reading all the mythic references to the moon, for instance, and by the next day I have a story or at least the feeling for a story.
A two volume set that pays less attention to the more commonly known Greek & Roman myths and focuses on African, Carribean, Asian and Native American beliefs. Full of great information that you don't often see in other dictionaries or encyclopedias.
These were really hard to find and they make me very, very happy. I get lost in these often, and always walk away with some new idea that I'd never heard of previously. I wish these were more available, because they're just awesome.