Those programmers into a bit of nostalgia may be interested in various early Apple documents made available on line by the Computer History Museum. Notably, these include the Apple II DOS source code, in M6502 assembler. The code is a reminder of just how hideous and painstaking it must have been to have to code something such as a disk operating system in what is reckoned to be a relatively short space of time.
Other hand-scribbled planning/specification documents released in the bundle will make programmers feel more relieved about the contents of their own notebooks.
For those wishing to pore even further over historic assembly language with that feeling of "How on Earth did that ever work?", you may also wish to take a look at the Spectrum ROM Disassembly (I have fond memories of having had this in book form at one stage: with the same feeling of disbelief!).