This book is several years old and tons of artists and websites have recommended it already, but I’m going to put in my two cents anyway in case you happened to have missed this gem.
Making Comics by Scott McCloud is just what the title suggests – a book about making comics. What makes this book especially fun is that entire book is written and drawn in a comic format.
Even if you’re not interested in drawing comics, it still has a lot to offer about storytelling and basic art principles. Any illustrator can learn a ton from this book. McCloud explains all the topics simply, yet interestingly, and includes different exercises and techniques to help the reader absorb the concepts along the way.
You can get a good idea of the format of the book on Scott’s website where he has posted a small sample, called Chapter 5 1/2. It expands upon Chapter 5 in his book “Tools, Techniques, and Technology”. It includes a tutorial and notes that cover optimizing color art for the web.