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: