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 2.6 source code is available at http://lxr.linux.no.
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, because it does not refer to any specific architecture. Some pertinent information about 80 × 86 GCC's Inline Assembly and gas, the GNU assembler invoked by GCC, can be found at:
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.
The newsgroup comp.os.linux.development.system is dedicated to discussions about development of the Linux system.
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 ...