Best Resources to Learn about Linux Kernel Internals

The Source

The best resource is the kernel source.

I prefer reading the cross-referenced lxr, which makes it easy to follow the code.
Linux Networking Subsystem: Desktop Companion to the Linux Source Code. A commentary Linux networking code.
The Linux Kernel & the File System Subsystem : An Architectural Overview. Talks about VFS in the Linux.
Code Commentary On The Linux Virtual Memory Manager by Mel Gorman.

Obviously, it’s not that easy to dive into thousands of lines of code. I suggest starting with reading the books explaining Linux kernel in general.

Understanding Linux Kernel. This is one of the first books to provide in-depth explanations. Vastly improved over multiple editions, the current one is a very good read.
Robert Loves’ book. Love is a core developer, who implemented pre-emptive kernel and other features. I haven’t personally read this, but I have seen good reviews of this.
If you are specifically looking for networking aspects, this is an excellent book on understanding linux networking internals.
For device drivers, similarly, this book is very useful.

LWN’s kernel page has lots of great articles explaining kernel internals
TLDP’s TLK. Somewhat outdated, but useful.
Linux Journal’s KernelKorner has some excellent articles, most of which are freely available online.
The Linux Kernel Hackers’ Guide, compiled by Michael K. Johnson of Red Hat fame. Includes among other
documents selected Q/A’s from the linux-kernel mailing list.

The LinuxKernel HOWTO by Brian Ward.
A completely new Kernelhacking-HOWTO at http://www.kernelhacking.org/.
Various Kernel HOWTOs on specific questions, such as the BogoMips mini-HOWTO by Wim van Dorst.
Various Linux HOWTOs at TLDP

There’s a huge index of links at http://jungla.dit.upm.es/~jmseyas/linux/kernel/hackers-docs.html that has links to many resources.
Another list of links maintained by Chris Gould. Mostly unsorted