I've used How to Draw Comics the Marvel Way, The Art of Drawing Manga, and Anatomy for Artists (by Barrington Barber) as my guides.
As an art student myself, I'd like to suggest that when you are going through material about drawing, try to stick with the stuff about realistic rendering as much as possible; such as Anatomy for Artists. I'd even go so far as to recommend that you shelve the comics and manga books until you hit your stride with the realistic stuff.
Mind you, I have nothing against manga or comics. Hell, I'm quite the anime geek actually. I only say that because I had made the mistake of starting to learn how to draw from things like comics and manga myself. In my case, the result was that I wound up learning the style rather than learning the skill of drawing. So when I took my first drawing class in college, I was basically back to square one. Furthermore, I've seen way too many of my fellow art students fall into that trap as well. They spent years just drawing manga or comics and they had allowed it to represent almost the entirety of their artistic scope. If you ask them to do something different, they couldn't. At least not well. Hence why I'm telling you to look at realistic anatomy books, or even draw from life as much as possible while you learn.
Another suggestion I'd make is to start playing around with shading. Line work is always good, but when you really examine features such as the nose, eyes, and mouth, you'll find that it's all alot more about shading than it is about line work. But you also need to be careful not to cover everything in a uniform shade. Shadow effects tend to utilize a very broad range of values.
Aside from that, all I can think to say is keep it up! If you get frustrated, always step back and take a breather and remember that achieving serious development takes time. And remember to really examine and measure the relationship of physical features, you'll be surprised as to how much you'll learn.
