If you want something more hands on you can try "FreeBSD Device Drivers: A Guide for the Intrepid" by Joseph Kong. It is much easier to read, and my guess is you ran into it while making those BSD patches.

He also has a short book on building rootkits for BSD, which is a very fun read and demonstrates how things like system calls work.

If you want a guide to reading actual kernel code, here is a great resource explaining where some of the foundational pieces are. With a tool like cscope, reading the FreeBSD kernel is actually pretty easy.

http://www.watson.org/~robert/freebsd/reading/

Hope that helps!