The best way to learn about almost any computer program is by using it. Most people find that reading a book without using the program isn’t beneficial. The best way to learn about Unix and GNU/Linux is by using them. Use GNU/Linux for everything you can. Feel free to experiment!
Debian isn’t as intuitively obvious as some other operating systems. You will probably end up reading at least the first few chapters of this book. GNU/Linux’s power and complexity make it difficult to approach at first, but far more rewarding in the long run.
The suggested way to learn is to read a little, and then play a little. Keep playing until you’re comfortable with the concepts, and then start skipping around in the book. You’ll find a variety of topics are covered, some of which you might find interesting. After a while, you should feel confident enough to start using commands without knowing exactly what they do. This is a good thing.