Well I believe IPTables is probably definitely what you would like to use in this case. There are two books that I know of out there.
Linux Firewalls - It is mainly geared towards firewalls but has a lot of information on IPTables as far as setting up multiple Class C networks behind one router.
Linux Routers - I have no read this so I really can't comment on it, but it has gotten good reviews from Amazon.
I hope this helps