Comment by jart

Rui is writing a book for the chibicc compiler in the cosmo codebase. I should probably write a book on system interfaces since there's no school for it. I had to go straight to the primary materials, i.e. the source to pretty much every existing kernel and libc along with the historical ones in order to understand the origin of influence. That's what helped me have a razor sharp focus on the commonalities which made this project possible.

So I'd say that the SVR4 source code would be a good place for you to start. It's like ambrosia and once you've read it you can always tell by reading modern code which developers have and haven't seen it. There's also the Lions' Commentary on Unix. I highly recommend Richard W. Stevens. The last book on the required reading list is BOFH.