Megamon 64 by Mark Schultz is not only a great ML Monitor, it is also very well documented. Docs are on disk.
http://www.commodore.software/downloads/download/56-machine-language-monitors/942-megamon-64

Megamon has a single-step feature (unusual in memory-resident monitors)
https://www.lyonlabs.org/commodore/onrequest/index.html