The Internet Sacred Text Archive is a very popular website which contains a huge free archive of books about religion, spirituality, folklore, mythology, and many more related areas. All major religions are covered: Christianity, Islam, Buddhism, Confucianism, Hinduism, Judaism, Mormonism, Shamanism, Shinto, Sikhism, ...
Almost all books are in html format for online reading only, but a few of them are available for download as zip-ed files.
