The Linux commmunity has done a kick-ass job in contributing quality documentation, in particular the HOWTOs by the LDP (Linux Documentation Project). Listed below the ones I got a lot of value from reading.

Linux on Laptops

Not part of the LDP but a kick-ass site that has HOWTOs for installing Linux on every major laptop vendor and model. Maintained by Kenneth Harker. The only knock on the site is that it doesn't get updated for months at a time.

The Unix and Internet Fundamentals HOWTO

The basics on how a UNIX system works. A well-written and easy-to-understand overview. Great explanation of system startup (from bootstrapping through the various run levels). By Eric Raymond.

Linux Partition HOWTO

Explains what partitions are and how to set up your system. By Tony Harris and Kristian Koehntopp.

Large Disk HOWTO

Explains how disks are used by Linux and BIOS limitations. By Andries Brouwer.

The Linux XFree86 HOWTO

Concise guide for getting X to work. By Eric Raymond.

X Window System Architecture Overview HOWTO

Describes the architecture and evolution of X. By Daniel Manrique.