This is not a rule, but a general conception. A few good security books are a good thing, although they tend to be out of date rather quickly. But they are great if you are just starting out, and they provide plenty of background information, and show you what to look for in future exploits or attacks. Although most Linux systems are setup with standard security in mind, they are also geared towards running alot of services default, to save the new user some grief I suppose. But the more windows you have in your house, the greater the chance of someone breaking one of them increases.
Less services is better.
For home systems, this is a very good step to start out. For business and to those that need more services, they should be a little more experienced. The following applies primarily to Slackware. One of the first things you should do is evaluate what you are doing to be doing with the system. Once you determine what you want, you can now get rid of the services you won't be using.