🚀 go-pugleaf

I can't suggest any specific source, but a good principle is to find the
oldest version of Principles of Ops, preferably an early /360 version from
the 60's. Although it will omit the newer architectural 'enhancements', the
fact that things were simpler back then makes the old manuals much more
readable (and shorter). If you do use any version of Principles of Ops, skip
most of it the first time and concentrate on reading about the basic kinds of
CCWs and how they are hooked together to make channel programs. Read about
SIO and HIO/HDV, but ignore all of the details of the various condition codes.
Do try to learn about CSW status flags, especially CE, DE, UC, and maybe UE.
Then pick a Components Description manual for a device you know something about
or want to try and program. It will explain in detail the CCWs for that device
as well as the CSW codes and SENSE information (look only at the first byte of
the latter). Many of these manuals (certainly the disk ones) include examples
of 'standard' channel programs. Good luck.