http://www.atariarchives.org/mlb/ -
Machine Language For Beginners
Classic 6502 beginner's book. Some experience with old-fashioned BASIC and 8-bit microcomputers is assumed.
http://ericclever.com/6500/ -
6502 - the first RISC µP
A reference card for the 6502. A more detailed reference than the one included in JBit-QS.