[Smalltalker-80]

Next to a book on 6502 assembly, you can google "apple 2 original system manual" and download it. I contains ROM assembly source code and schematics.

[lakkal]

"reference manual" I think is what you mean. This is a link to a PDF pf the reference manual that includes the ROM disassembly: http://cini.classiccmp.org/pdf/Apple/Apple%20II%20Reference%...