Hi there, and welcome to LQ.
The best way to learn about GNU/Linux is probably a balanced combination of reading and practice. The more theoric knowlege you have, the better you will be able to do what you want with your system. On the other hand, practice will reinforce theory by completing it and by posing new problems. That's a dynamic process. Regarding documentation, there are many online sources available about GNU/Linux: books, forums, wiki pages, software manuals, and so on and so forth.
A few examples:
The Linux Documentation Project
Introduction to Linux - A Hands on Guide
Bash Guide for Beginners
What is free software?
The distributions themselves are the main, hands-on learning tool. They also store documentation on your system, about the installed software. Regarding the best distribution to start with, there is no ultimate answer, and many discussions are already available on these forums. You say that you are interested in Slackware, so you might start with it right away (see the links below). Otherwise, search about the existing distributions and compare their genealogy, features and target, and then try one (or more) of them. The links posted above by coralfang
may be a goot starting point. I suggest that you also look at this page
There is no such thing as an official "Beginner / Intermediate / Advanced" classification of distributions. Many people find the so-called "user-friendly" distributions to be easy to use, whereas others find them difficult, because of the massive layer of automation they ship with, often preventing the user to have full control on the system. On the other hand, the so-called "difficult" distributions are sometimes perceived as easier to use, because they don't get in the way by imposing unwanted automation, thus allowing full control, and they are usually less complex and more stable. The final choice is up to you.
Because you are asking about Slackware:
The official website
Slackware Linux Essentials
The revised slackbook project
Slackware Documentation Project
The official forum
(right here on LQ)