In addition to the Linux Documentation Project Home Page: http://www.linuxdoc.org/, there are many pages that provide beginning and advanced information about Linux.
These two pages provide a good starting point for general Linux information: Linux International's Home Page, at http://www.li.org/, and the Linux Online's Linux Home Page at http://www.linux.org/.
Both of these pages provide links to other sites, information about general information, distributions, new software, documentation, and news.
Documentation for kernel developers is on-line: http://kernelbook.sourceforge.net/.
The tutorial, Unix is a Four Letter Word..., is located at http://www.linuxbox.com/~taylor/4ltrwrd/. It is a general introduction to Unix operating systems and is not Linux specific.
Additionally, here is a certainly incomplete list of Web pages devoted to Linux:
Searching for "Linux" on Web Search Engines, like Yahoo! (http://www.yahoo.com/), Altavista (http://www.altavista.com/), or Google (http://www.google.com/) will provide copious references to Linux Web sites. Further information about about
Web search engines is in the Web and Internet Search Engine Faq: http://www.infobasic.com/pagefaq.html.
Refer also to the answer for: "What Other FAQ's and Documentation Are There for Linux?"