A guide to programming the 6502 - originally hosted on ntlworld.com
This guide comes in the form of a single, self-contained, fully cross-referenced, HTML file.
For a comprehensive guide, you may consider buying a copy of "programming the 6502" by Rodnay Zaks https://www.amazon.co.uk/Programming-6502-Rodney-Zaks/dp/B009X0EL4O/ https://archive.org/details/Programming_the_6502_OCR
I have also included the C source code which I used to generate the instruction table ...it's fairly clean code, so might be a good starting place for a disassembler or emulator.