If you are looking to learn operating system concepts using RPi there is a new textbook by Prof. Vanderbauwhede, under ARM textbook initiative [1].
I think for educational purposes it will be very interesting and useful if someone develop from scratch a simple System V UNIX for RPi based on Bach's description in his seminal book on "The Design of the UNIX Operating System" [2]. The book also covers multi processors topic as supported by modern platforms such as RPi.
[1]https://www.arm.com/resources/education/books/operating-syst...
[2]https://github.com/suvratapte/Maurice-Bach-Notes/blob/master...