These documents tend to be more step-by-step, and typically provide just what information is needed to perform a task, rather than going deep into theories of operation. They are excellent sources of information, and are usually more current than printed materials.
A HOWTO document designed for DOS and Windows users who want to learn more about Linux
A HOWTO document that covers numerous pieces of hardware, their support in Linux, and potential problems.
A HOWTO document covering all aspects of Linux over Ethernet
A HOWTO document covering Linux firewalls
A HOWTO document covering the Masqerading (address translation) in Linux routers and firewalls
A HOWTO document covering the IPCHAINS utility which is used to create the actual firewall rules