It looks like your about my age, and your on the right track. I want to adress some things first
there is nothing wrong with being a "dabbler" this is actually a good thing, the best artist is a well rounded artist. Also the worste thing you can do is start and stop. Keep on going, always draw. Draw in class, draw while your waiting for your Mcdonalds order to come through, draw while on the can. This is a best thing that you can do is draw from the second minute you wake up to well into the night. A person that likes to create art draws an hour a day, a good artist draws for 2 hours a day, a great artist is the person that has an indent of a pencil in their hand.
Now to what your doing and posting. Its great
Keep going through that book, its a good one. It will help you see and not just draw. One thing that would help you out is to post some goals that you want to achieve, such as learning colour or being able to draw a figure from imagination.
BTW dont worry about lack of replies, if you start posting in other sketchbooks and update this thread regularly and they will be pouring in
Overall your on the right track. Also dont be afraid to just attack the paper, I know you have crazy imagination in there let it free man.
Daily sacrifices to the ART GODs is manditory <><> I do it myself <><> a day without drawing is like a day without sex <> well not that bad, but bad- Mentler
Be the change you want to see in the world- Mahatma Gandhi
Art is not a substitute for nature, but an expression of feeling produced in the consciousness of the artist- Harold Speed