Regarding guides specifically, FreeBSD has exceptional resources:
FreeBSD Handbook[0]
FreeBSD Porter's Handbook[1]
FreeBSD Developers' Handbook[2]
The Design and Implementation of the FreeBSD Operating System[3]
Not to mention that the FreeBSD man pages are quite complete. Granted, I am biased as I have used FreeBSD in various efforts for quite some time and am a fan of it. Still and all, the project's documentation is a gold standard IMHO.0 - https://docs.freebsd.org/en/books/handbook/
1 - https://docs.freebsd.org/en/books/porters-handbook/
2 - https://docs.freebsd.org/en/books/developers-handbook/
3 - https://books.google.com/books/about/The_Design_and_Implemen...