This might be a good place to start.
The FreeBSD No Starch book just came out a year and a half ago and the Handbook is constantly updated so I don't know why you think they're outdated.
In addition, if you really want to get into the bowels of the FreeBSD operating system
What I am doing right now is reading this book alongside with the source code of 11.4.
On and off I build the kernel and try to understand it’s output and the overall make infrastructure.
FreeBSD is a fantastic OS.
I recommend getting the books The Design and Implementation of the FreeBSD Operating System & Absolute FreeBSD
LOL it seems interesting to me. I'm reading https://www.amazon.ca/Design-Implementation-FreeBSD-Operating-System/dp/0321968972/ref=dp_ob_title_bk right now.
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Maybe its good in theory, and not in practice.
https://news.ycombinator.com/item?id=11655472
Do you know how a queue/an event loop works? It turns out that all of OS is basically just a combination of these to a large extent.
Also check out these books http://pages.cs.wisc.edu/~remzi/OSTEP/
and https://www.amazon.com/Design-Implementation-FreeBSD-Operating-System/dp/0321968972/
Forget all the other OS books.