Religion is certainly a huge subject to cover! This index will contain books pertaining to all major religions still in practice, from Buddhism to Christianity to Paganism and many more, plus the deities and symbols associated with those religions. It will not cover ancient religions and deities or religions restricted to certain ethnic groups, such as the Mayans. (These may be found on the index page for that particular ethnic or ancient group.)
This is another one of those pages where my decision on what to and not to include will probably change. In general if it is associated with the sacred or spiritual, philosophical or esoteric/occult, or even morals in a spiritual sense (not a social sense), it will most likely be found here. Non-fictional books, for instance those which are about spiritual or religious practice or philosophy will certainly be here, but so will fictional books that use religion or spiritual belief strongly as a theme, and that includes atheism and agnosticism. There's lots here to explore and enjoy!