Marshall Kirk McKusick's FreeBSD Intensive Code Walkthrough: https://www.mckusick.com/courses/advdescrip.html
Also, The Design and Implementation of the FreeBSD Operating System (2nd Edition): https://www.amazon.com/Design-Implementation-FreeBSD-Operati...
Thirdly: grab a copy of FreeBSD (or OpenBSD) and (a) set it up in VirtualBox and SSH it into locally (b) use an old ThinkPad. Then grab the source code of the base system. Build and install it. And start reading code of things like usr.bin/grep/grep.c
The Design and Implementation of the FreeBSD Operating System
FreeBSD has for years had some features that are only now reaching 'mainstream' popularity (e.g. jails, which containers are based on). This books explains all that in quite a bit of detail and manages to stay high level enough not to get boring with the details but gives you enough detail to have a pretty detailed understanding.
It's also one of the few books that talks about networking in very practical terms (and not like OSI layers and whatnot) which makes me say that it's also possibly one of the best books on networking (you learn quite a bit about networks from how OS's work with them).
It also has the hands down best file system around (the book also talks about that).
And unlike large parts of linux, the kernel source is actually pretty legible.
I've been making my way through The Design and Implementation of the FreeBSD operating System, which is a bit denser and more formal. It was nice to read about some of the same ideas in a different implementation. I'd recommend the book to anybody who enjoyed the post.
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