The primary source of free documentation on Linux is the Linux Documentation Project (LDP ). The main LDP web site is http://www.tldp.org, but there are many mirror sites around the world, one of which may be closer to you or less busy.
The documentation in the Linux Documentation Project is organized into several types. The Guides are long, often book-length, manuals covering in detail such larger topics as networking. The HOWTOs are medium-length documents covering specific tasks, such as configuring a sound card. For smaller tasks on specialized topics that don't justify a full HOWTO, there are mini-HOWTOs. Finally, there are a number of FAQs that answer frequently asked questions on Linux.
The LDP documents are provided in a number of different formats, including HTML, plain text, PDF, and PostScript. Many of the documents have also been translated into different languages by a team of volunteer translators.