[This is a fast-access FAQ excerpt.]The Oxford Companion to the English Language (ed. Tom McArthur,
Oxford University Press, 1992, ISBN 0-19-214183-X) is an
encyclopaedia with a wealth of information on various dialects, on
lexicography, and almost everything else except individual words
and expressions. Success With Words (Reader's Digest, 1983, ISBN
0-88850-117-X) is especially suitable for beginners.
[For reviews of other English Language books, see Michael
Quinion's World Wide Words site.]