For anyone who's serious about becoming an illustrator I recommend Charles Bernard's tutorial videos on http://www.onlineartacademy.com/
While his style is somewhat old-fashioned, he knows all about the rules of form, perspective, design and composition, how to paint folds, lighting the head to get various moods and many, many other topics. The videos were a real eye-opener to me regarding how deep the subject of illustration can be.
He explains everything in a clear and concise manner, and he isolates each subject so that you don't end up getting confused.
If you can afford to pay for the vids I say you should go for it.