The Adobe Illustrator CS Wow! Book
Sharon Steuer
Peachpit Press
I had some Illustrator experience before picking up this book, which is important: this gives you the basics and tells you what to do when you're beyond the basic level. The author doesn't just tell you what to do, but WHY to do it. The book is full of lesson plans covering a broad scope of Illustrator functionality, for example how to use the "divide" command to create nice abstract graphics effects. Similarly, I never really had a good grasp of the usefulness of blends and gradient meshes before completing the lessons. I also got a taste of functions that I don't often use, such as custom brushes and 3D effects. I might not use them more often in the future, but if I ever need them, it's nice to know they exist.