The Linux Kernel API

Documentation on many of the kernel API calls, with some searching I Found some excellent pages on this.

http://www.gnugeneration.com/books/linux/2.6.20/kernel-api/

http://tldp.org/LDP/lkmpg/2.6/html/index.html

http://kernelnewbies.org/KernelHacking

http://www.advancedlinuxprogramming.com/alp-folder

Linux USB drivers — Understanding and developing Linux USB drivers
Embedded Linux optimizations — Optimizing the Linux kernel and applications for speed, size, RAM, power and cost.
http://free-electrons.com/articles

Advanced Linux Programming
http://en.wikibooks.org/wiki/Linux_kernel