The Linux Documentation Project (LDP) used to be an all-volunteer project that maintains a large collection of GNU and Linux-related documentation and publishes the collection online. It began as a way for hackers to share their documentation with each other and with their users, and for users to share documentation with each other. Its documents tend to be oriented towards experienced users such as professional system administrators, but it also contains tutorials for beginners.
Today, the LDP serves over 475 documents contributed by even more authors. About a dozen of them are book length, and most of those are available in print from major technical publishers including O'Reilly.
The LDP published many HowTo documents, which instruct a user on the specific steps to take to achieve a desired goal. These goals are sometimes very specific, such as configuring a particular modem, and sometimes very broad, such as how to administer a network for an ISP.
Very broad topics were covered in the guides, which are book-length documents, usually on broad subjects such as security or networking.
Much of the LDP collection is licensed under the GNU Free Documentation License (GFDL). Many other licenses are also used, as long as they are freely distributable. Current policy recommends the GFDL.
Linux Network Administrator's Guide is one book in the series.