Linux Project Guides are basically books, however they are generally found in online in a variety of formats -- HTML, plain text, DVI, and PostScript are common methods.

• Installation and Getting Started Guide
  Author(s): Eric S. Raymond
  Non-English translations are also available.
  

• The Linux Kernel Hackers' Guide
  Author(s): Michael K. Johnson
  Format: Available only in HTML.
  Description: This manual is a guide to the Linux kernel, ranging from concepts to development. If you're interested in writing a device driver, or just general kernel hacking, this is the book for you.

• The Linux Kernel
  Author(s): David A. Rusling
  Format: Also available in HTML (tarred and gzipped) DVI, LaTeX Source, and PostScript.
  Description: The kernel is at the heart of the operating system. This book is a guide to how the kernel fits together, how it works; a tour of the kernel.

• The Linux Kernel Module Programming Guide
  Author(s): Ori Pomerantz
  Format: Also available in HTML (tarred and gzipped) DVI, LaTeX Source, PDF, and PostScript.
  Description: This document is for people who want to write kernel modules.

• The Linux Network Administrators' Guide
  Version: 1.0
  Author(s): Olaf Kirch
  Format: HTML (tarred and gzipped) DVI, LaTeX Source, Plain Text, PDF, and PostScript. Non-English translations are also available
  Description: This guide supplements the System Administrators' Guide and cover all of the diverse issues of networking under Linux, from UUCP to serial connections to TCP/IP. Many Linux users won't have access to such a network, so this information is in a separate manual. It contains an intro to TCP/IP and UUCP (for those who have never used such networks before, lots of background information), TCP/IP, UUCP, SLIP, and DNS configuration, configuration of mail systems such as sendmail and Smail, setting up NNTP and news, and NFS.

• The Linux Programmer's Guide
  Version: 0.4
  Author(s): B. Scott Burkett, Sven Goldt, John D. Harper, Sven van der Meer and Matt Welsh
  Format: Also available in HTML (tarred and gzipped) DVI, LaTeX Source, PDF, and PostScript. Non-English translations are also available.
  

• The Linux System Administrators' Guide
  Version: 0.6
  Author(s): Lars Wirzenius
  Format: Also available in HTML (tarred and gzipped) LaTeX Source, PDF, and PostScript. Non-English translations are also available.
  Description: This is the third book in the main LDP series, and assumes knowledge of everything in the Installation and Users' Guides. It will cover all of the aspects of keeping the system running, handling user accounts, backups, configuration of the system, installing and upgrading software, and more. Whereas some of this information is in the Installation Guide (just to get the system off the ground) this book should be much more complete.

• The Linux Users' Guide
  Version: beta-1
  Author(s): Larry Greenfield
  Format: Available only in DVI, LaTeX Source, PDF, PDF, PostScript, and PostScript. Also available in Non-English translations
  Description: This book covers all of the user-end aspects of Linux, from sitting down at the first login session to using complex tools such as gcc, emacs, and so on. It assumes no previous Unix experience, so not only will it serve as an introduction to Linux, but to Unix in general as well. This manual won't cover system administrator tasks (i.e. anything that needs to be done as root) - it's for the J. Random User who has a working Linux system sitting in front of them.

In addition to online documentation, there are a wide variety of Linux and Linux-related books and other publications available.