Nasm "Hello world"

PC Assembly Language is resource on assembly programming. It's modern, free, and covers a variety of topics. It uses NASM and the examples are available for Linux (among several other systems).

hello-world-nasm-linux
64-bit-hello-world-in-linux-assembly-nasm

Docs

http://www.nasm.us/docs.php

x64
Gentle Introduction to x86-64 Assembly
64-bit Linux system call numbers
Intel® 64 and IA-32 Architectures Developer's Manual: Vol. 2B