Firewall Rules Overview – a good summary of some of the basics: https://cloud.google.com/vpc/docs/firewalls
Fine-tuning Firewall Rules: 10 Best Practices – reinforces a lot of the ideas mentioned in this chapter and even has a few new ideas: https://www.esecurityplanet.com/network-security/finetune-and-optimize-firewall-rules.html
Network Address Translation (NAT) – an overview of NAT: http://www.rhyshaden.com/nat.htm