Linux iptables Pocket Reference is a really great little book, and although it’s been written a good few years ago, most of the explanations still apply.
iptables is a great way to manage all your needs running a Linux-based gateway server: filtering and connection blocking (firewall), proxying, packet forwarding or one of the network address translation (NAT) schemes – it’s all possible via straight-forward and easy to remember interface.
Read the full review here: Linux iptables pocket reference.