Learning from others
Learning from other programmers is very useful. This page will contain a list of commercial, PD or shareware titles coded in assembly language where, with a bit of work, you can extract the source code to the majority of the game in question!
D-Day Populous II (.PRG will need unpacking as it's using Rob Northern's pack routine)
Commercial (with problems)
9 Lives - original .PRG is stored in raw format on the disk but the labels are intact, so you'll need to re-create the 28 byte header that's on all .PRG files using your brain!
Cool Spot (uses an Amiga format file .. so might not be possible)