I was an Illustrator for a few years, drifted off into other work for many years and decided to return to the profession. Unfortunately, as you may know already, knowledgable, helpful, advice either in an Art School setting or out there in the world is thin on the ground. If you're in that position, then buy this book.
This book covers both the why, the aethetic, the how, technique, and most importantly if you're anywhere near having sorted the first two; how to market yourself. There is tons of practical advice on portfolios, presenting - and it's all delivered from a professional point of view and it rings true.
This is not a book for sunday painters. This is a book for the professional and the aspiring professional and I can't recommend it more highly. Oh, and if you're saying "yeah..." google me.