I don't recommend books, I recommend experience. Set up a couple of boxes with nothing important on them and start up a bunch of services and let them get attacked. I would do that with no firewall, but install an IDS such as snort so you can see what is going on. That is a good way to learn. Just make sure that nobody takes control of your box, you don't want them launching attacks from it. I would just reinstall the OS like every week, it is good experience, and each time set it up more secure, plug something up that was attacked the last time. Over time you will start to recognise certain things and learn to protect yourself from them. Just my 2 cents.