using netfilter hooks in LKM's

hi!

my question is related to programming with netfilter hooks and using
them with LKM's in linux.

i found some material about it in the actual phrack issue.
now i am interested in programming LKM's especially to make use of the
netfilter hooks and netfilter features in general.

i studied some of the kernel sources in net/ and net/core.
for me, the sources were not very informative because
ive never done network programming at such a low level.

the way the kernel works, especially when doing networking stuff,
fascinates me.
well, where can i find some more specific material on programming things
like that?
is there a good book you can recommend,
that shows me the networking abilities of the linux
kernel?

thank you alot!

cya