The Linux Kernel Hackers' Guide

The Linux Kernel Hackers' Guide
http://www.tldp.org/LDP/khg/HyperNews/get/

Due to the fact that nearly every post to this site recently has been either by rude cracker-wannabes asking how to break into other people's systems or a request for basic technical support, posting to the KHG has been disabled, probably permanently. For now, you can read old posts, but you cannot send replies. In any case, there are now far better resources available.
Go get the real thing!
Alessandro Rubini wrote Linux Device Drivers, which is what the KHG could have been (maybe) but isn't. If you have a question and can't find the answer here, go get a copy of Linux Device Drivers and read it--chances are that when you are done, you will not need to ask a question here.
Run, don't walk to get a copy of this book.

The Linux Kernel
Go read The Linux Kernel if you want an introduction to the Linux kernel that is better than the KHG. It is a great complement to Linux Device Drivers. Read it.
Table of Contents
Tour of the Linux Kernel
This is a somewhat incomplete tour of the Linux Kernel, based on Linux 1.0.9 and the 1.1.x development series. Most of it is still relevant.
Device Drivers
The most common Linux kernel programming task is writing a new device driver. The great majority of the code in the kernel is new device drivers; between 1.2.13 and 2.0 the size of the source code more than doubled, and most of that was from adding device drivers.
Filesystems
Adding a filesystem to Linux doesn't have to involve magic...
Linux Memory Management
A few outdated documents, and one completely new one by David Miller on the Linux cache flush architecture.
How System Calls Work on Linux/i86
Although this was written while Linux 0.99.2 was current, it still applies. A few filenames may need updating. find is your friend--just respond with the changes and they will be added.
Other Sources of Information
The KHG is just one collection of information about the Linux kernel. There are others!