The original programmer's guide for the '2600 is the Stella Programmer's Guide by Steve Wright.

Although a wonderful technical resource, some caution is advised when using the guide. Modern programmers know far more about the machine than was known in 1978, and in some cases the manual has proven to be slightly incorrect. Rely on data from this Tiki in preference to that shown in the guide.

PDF: Stella Programmer's Guide

Alternate Sources

Stella Programmer's Guide
Original Typewriter Version
Unofficial HTML Version