There is quite a bit of documentation around the internet on the linux boot process, but Gustavo Duarte I think did an excellent job describing this in a clear and concise way. He also has several links to the Linux kernel source code and describes what is occurring step-by-step through the bootstrap phase all the way to the execution of /sbin/init.
His first entry lays the foundation of the basis of the x86 Intel chipset, memory map, and logical motherboard layout. This provides a basic understanding about the traditional hardware motherboard implementations.
Next, he describes BIOS initialization, and loading of the MBR. This briefly touches on the boot loader which starts the Linux bootstrap phase.
Finally, the kernel boot process is detailed with links to C and Assembly source code, with a brief narrative of exactly what is happening.
This was an awesome description of the early-on start up and initialization phases of hardware and bootstrapping of the O/S. Gustavo provides a great description of real-mode and protected-mode CPU states.