Thank you for helping me with the graphics card thread - I clicked on your sketchbook and think I can help you too! I specialise in illustration and drawing animation by hand so can give you some tips straight from art school -
Steps -
Find out where you are
Find out how to practice
Experiment
Finding out where you are:
Your rendering is exceptional, you have shown that you can draw from reference very well indeed. Therefore this first step is often the hardest. To find out where we were (even after 5 years A Levels and diplomas in art) the first thing they did in university was to make us draw something from memory. In your case I guess it will be the human form, which isn’t easy.
The purpose is to find out what you actually know about the standard form before you sit in front of it. After doing this exercise you will be able to clearly identify forms that you know well and others that you don’t. Its extremely valuable and will help you improvise in the future. After all, if you know the form like the back of your hand - you can draw it doing anything from any angle.
Finding out how to practice:
Photographs and diagrams are very good reference. You can do that very well now however, so I’d encourage you to draw from life. Start with some still life then move on to life drawing.
Definitely go to life drawing sessions. I still go to them even though I’ve left University. Just search in your local area and you should find something. They’re usually v inexpensive.
Until you can do that a very good (free) reference site is posemaniacs.com . Look for the tool on there that gives you a time limit of 30 - 90 secs per pose (thats typically what you’ll get in speed life drawing).
Experiment:
You look as if you can use pencils very well. To build your knowledge and improve your technique try other mediums (paint, pen/ink, charcoal etc). If you go to life drawing classes, they will probably encourage you to do this too.
Study colour, drapery, hair, gestures and weight as you go along. Also look into illustration a little if you want to eventually make stylised imaginary characters.
Affordable reference books for drawing I can recommend are anything by Barrington Barber. I used him when I first started out and still use him now. In terms of draftmanship he covers everything. A popular one is also Burne Hogarth, although due to his articulation I think that his books are overrated.
Good Luck!