Linux source code
The official site for getting kernel source can be found at http://www.kernel.org.
Many mirror sites are also available all over the world.
A valuable search engine for the Linux 2.4 source code is available at http://www.tamacom.com/tour/linux/.
GCC manuals
All distributions of the GNU C compiler should include full documentation for all its features, stored in several info files that can be read with the Emacs program or an info reader. By the way, the information on Extended Inline Assembly is quite hard to follow, since it does not refer to any specific architecture. Some pertinent information about 80 x 86 GCC's Inline Assembly and gas, the GNU assembler invoked by GCC, can be found at:
http://www.delorie.com/djgpp/doc/brennan/brennan_att_inline_djgpp.html
http://www.ibm.com/developerworks/linux/library/l-ia.html
http://www.gnu.org/manual/gas-2.9.1/as.html
The Linux Documentation Project
The web site (http://www.tldp.org) contains the home page of the Linux Documentation Project, which, in turn, includes several interesting references to guides, FAQs, and HOWTOs.
Linux kernel development forum
The newsgroup comp.os.linux.development.system is dedicated to discussions about development of the Linux system.
The linux-kernel mailing list
This fascinating mailing list contains much noise as well as a few pertinent comments about the current development version of Linux and about the rationale for including or not including in the kernel some proposals for changes. It is a living laboratory of new ideas that are taking shape. The name of the mailing list is [email protected].
The Linux Kernel online book
Authored by David A. Rusling, this 200-page book can be viewed at http://www.tldp.org/LDP/tlk/tlk.html, and describes some fundamental aspects of the Linux 2.0 kernel.
Linux Virtual File System
The page at http://www.atnf.csiro.au/~rgooch/linux/docs/vfs.txt is an introduction to the Linux Virtual File System. The author is Richard Gooch.