One of my favourite books when I'm thinking or talking about Fantasy. It's a bit dated now, (it came out in '99), and yes there is an online version, but for all that I love the hardcopy edition. If you're interested in Fantasy, as a reader or a writer, then The Encylopedia of Fantasy is for you.
Warning! Just like Brewer's Phrase and Fable, you'll never just look up one thing, time will vanish on you, though you will have found out 10 cool things along the way.Encyclopedia of Fantasy by John Clute and John Grant