> however a disadvantage is that the reader needs to know or learn a new programming language first

This is a good point, and I have thought about it a lot before starting the book. What finally made my choose T3X is that its compiler is much smaller[1] than my smallest C-subset compiler and (IMHO) T3X is easier to learn or understand.

[1] SubC: 3815 lines, T3X/0: 2330 lines.

Of course you could start on CP/M without any outside tools, but then you would have to write your bootstrapping compiler in assembly language. Time-consuming, but certainly manageable. I doubt that it would be an interesting reading, though.