Chapters 3--6 contain useful information. Note that much of this documention is irrelevant---we are only running in “flat” addressing mode, and GAS does not support conditional moves, MMX, or SSE instructions. The overall document is 369 pages long. You probably don’t want to print it out.
Chapter 3 contains a detailed description of every instruction. Note that they list the operands in the reverse order from their ordering in GAS. This manual is 854 pages long. You definitely don’t want to print it out.
Contains lots of useful information on the interface between the hardware and the operating system, including cache and TLB specs, virtual memory, exceptions and interrupt handling, and processes.