Here are even more:

Info on SysV ABI:
http://www.x86-64.org/documentation/abi.pdf

amd64 registers:
http://www.logix.cz/michal/devel/amd64-regs/

x86 opcode and instruction reference:
http://ref.x86asm.net/

Thanks to genss from LQ.

All links are collected in the thread
on LQ:
http://www.linuxquestions.org/questions/programming-9/assembly-resources-4175573485/

Mitt