Starter Resources
Books
Classic Game Programming on the NES by Tony Cruise
Walks through every step of coding a NES game in assembly.
I AM ERROR by Nathan Altice
A combined history and architectural exploration of the Famicom/NES.
Programming the 65816 Including the 6502, 65C02, and 65802 by David Eyes and Ron Lichty
Classic text on assembly coding, technically written for 65816 (used by the SNES!) but with the 6502 sections clearly marked off.
Online Tutorials
Extremely well written, but as yet incomplete.
An old classic.
Interactive walkthrough teaching the basics of 6502 assembly. Very cool.
What it says on the tin, though more of an overview than an in-depth tutorial. I haven't tried it myself.