The Debian web site contains a large quantity of documentation about Debian. In particular, see the Debian GNU/Linux FAQ and the Debian Reference. An index of more Debian documentation is available from the Debian Documentation Project. The Debian community is self-supporting; to subscribe to one or more of the Debian mailing lists, see the Mail List Subscription page. Last, but not least, the Debian Mailing List Archives contain a wealth of information on Debian.
A general source of information on GNU/Linux is the Linux Documentation Project. There you will find the HOWTOs and pointers to other very valuable information on parts of a GNU/Linux system.