If you prefer text, you can find it in Chapter 1 of his book "The Design and Implementation of the FreeBSD Operating System": https://www.amazon.com/Design-Implementation-FreeBSD-Operati... (the chapter seems to be available in the book preview)
Eric S. Raymond also wrote a nice chapter on UNIX history in "The Art of Unix Programming": http://www.catb.org/esr/writings/taoup/html/historychapter.h...
There are books, such as The Design and Implementation of the FreeBSD Operating System (by McKusick): http://www.amazon.com/Design-Implementation-FreeBSD-Operatin...
There are papers, such as Jonathan Lemon's "Kqueue: A generic and scalable event notification facility" presented at Usenix 2001: http://people.freebsd.org/~jlemon/papers/kqueue.pdf
There are kernel interface man pages:
http://www.freebsd.org/cgi/man.cgi?query=SYSCALL_MODULE&sekt...
http://www.freebsd.org/cgi/man.cgi?query=hhook&apropos=0&sek...
There are examples referenced by the man pages:
Algorithms and Data Structures:
http://www.amazon.ca/Introduction-Algorithms-Thomas-H-Cormen...
http://www.amazon.ca/Algorithms-4th-Robert-Sedgewick/dp/0321...
Computer Architecture and Operating Systems:
http://www.amazon.ca/Computer-Architecture-Quantitative-John...
http://www.amazon.ca/Design-Implementation-FreeBSD-Operating...
Mathematics:
http://www.amazon.ca/Calculus-James-Stewart/dp/0495011606
http://www.amazon.ca/Introductory-Statistics-Prem-S-Mann/dp/...
http://www.amazon.ca/Introduction-Mathematical-Cryptography-...
Networking and Systems:
http://www.amazon.ca/Computer-Networks-5th-Andrew-Tanenbaum/...
http://www.amazon.ca/Database-Management-Systems-Raghu-Ramak...
http://www.amazon.ca/Distributed-Systems-Principles-Andrew-T...
Learn C as necessary.
The FreeBSD kernel book is worth a look too: http://www.amazon.com/Design-Implementation-FreeBSD-Operatin...
McKusick's 'Kernel Internals' class is based on this, and is well worth your time (though it's a bit pricey to purchase the videos on your own: https://www.mckusick.com/courses/advorderform.html)