General Reference: Encyclopedias

"a work that contains information on all branches of knowledge or treats comprehensively a particular branch of knowledge usually in articles arranged alphabetically often by subject"

From Merriam-Webster's Online Dictionary

• Artcyclopedia
  An encyclopedia featuring art and artists.
• Encyclopedia.com
  Scroll down to get beyond the advertising on this site.
• Info Please Encyclopedia
  A comprehensive resource of electronic information from the Columbia Encyclopedia, Sixth Edition. You can search by topic or keyword.
• Symbols.com
  Search by word or symbol description to find what the symbol means.