I started reading 'Introduction to Operating Systems Abstractions-Using Plan 9 from Bell labs' by Francisco Ballesteros and solving
exercises. It works fine with the newest version for the raspberry pi (single machine) and one already gets a good experience.

The most amazing thing is the code-you open a random file in /sys/src and you find readable, good code.
Most codebases have ugly spots and you see them when you skim through-but I haven't had this experience while reading
any Plan 9 code.