Classicist's Online Library

The links go to e-books at the Internet Archive.
You might gave to sign-up for a FREE account, to read them. 

• Dictionary of Greek and Roman biography and mythology  William Smith, Ed.
  Dictionary can be searched by keyword, but use the Table of Contents on the left to find an article faster.
• Dictionary of Classical Mythology: Symbols, Attributes, and Associations / Bell 
  Want to know which myths mention apples?  Look here. 
• Mythology / Hamilton
• A classical dictionary / Lempriere
  Good place to find obscure figures. Citations to classical works which mention the character.