I have found that a classical book
Flatland
A romance of many dimensions
With Illustrations by the Author, A SQUARE
(Edwin A. Abbott 1838-1926)
is published online.
As far as I remember, the first time I read about Flatland in one of Martin Gardner's books,
Another good classical book published online is
"Symmetry Groups and Their Applications
by Willard Miller
Academic Press, New York, 1972 " (out of print, published online by its author). The book is written from application point of view (mostly application to physics) and it contains quite a good material on the theory of finite groups.
It costs 374 USD at amazon.com on August 4 2005
Enjoy!