Linux source code browser: http://lxr.free-electrons.com/source/

Illumos source code browser: http://src.illumos.org/source/

Intel architecture manuals: 
http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html

John Levine, "Linkers and Loaders": http://www.iecc.com/linker/