Sounds like you actually want a general linux administration book, not something specific to Ubuntu. In all actuality, aside from a few distro-specific commands for installing packages, managing rc/init scripts, managing an ubuntu server is just like managing any other linux distro.
So, for a general linux administration book, I'd highly recommend the Rute Tutorial. Yes, it's been around for over 10 years now, but the basics of managing a system haven't changed. There is still a ton of great information in Rute, and what you learn from reading it will be applicable to any linux system you touch going forward.
For a quick guide on IPTables, I'd recommend IPTablesRocks. That will give you a good overview of IPTables' structure, and from there, you can either roll your own firewall rules or use one of the many IPTables "helper" scripts.