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.
Is the ebook of the textbook a regular PDF/epub/mobi or is it only viewable in their viewer apps? The page doesn't say, and I don't want to spend money on it and then realize it's not a real download.
A few people seem to have ported xv6 to arm.
https://github.com/david50407/xv6-rpi
https://github.com/houcheng/xv6-armv7/