I started with LDD [1], which is a book from a decade ago, but should still be relevant these days. And at some later time I found this treasure trove [2] hidden in FreeBSD docs, among which, the FreeBSD Architecture Handbook and the FreeBSD Developers' Handbook may be of special interest to you.

[1] https://lwn.net/Kernel/LDD3/

[2] https://docs.freebsd.org/en/books/