Make sure you have installed the documentations that come with Slackware (HOW-TO, Mini-How-To, FAQ, etc.). Start with your friends "man", "apropos" and "whatis".
Try reading that and see if it makes any sense.
If you need basic UNIX/Slackware docs:
If you need more advanced books:
For everything else, buy O'Reilly books. They are simply the best out there...
Also, do read the comments in the /etc/rc.d/* files: they actually explain a lot
Yes, I know, you wanted something simple... But what could be more simple than reading documentation that you already have on your system?
Managing UNIX/Linux/Slackware machines is 95% reading manuals & HOW-TOs, 3% changing configuration files and perhaps 2% swearing... Have fun!