Where should I start learning Assembly?

Linux Assembly Tutorial: http://docs.cs.up.ac.za/programming/asm/derick_tut/

Introductory Book: http://www.amazon.com/x/dp/0763772232/

Reference: http://www.intel.com/content/www/us/en/processors/architectu...

x86 ? This should get you started: http://www.asmcommunity.net/

Code by Charles Petzold [1] is a fantastic introduction. It isn't so much the nitty gritty "this opcode performs this operation, and these are all the tricks to making it do things, edge cases and things you should worry about" and more along the lines of "what opcodes should a CPU have, and how do those translate into electricity flowing through physical wires?" I feel like really thinking through that book made MIPS and x86 assembly much easier for me.

1 - http://www.charlespetzold.com/code/

Which assembly? x86, PowerPC, ARM, MIPS?

Personally my favourites are 6502 (http://skilldrick.github.io/easy6502/) and 68k (http://www.easy68k.com/) tho' neither of these are realistically of any commercial use.

I disagree. It diverges into HLA (high level assembly) which is pretty much a macro monoculture that is tied to this book and nothing else. I was rather disappointed with the book.