If you are searching for just the right book to buy for Adobe Illustrator, or any other graphics software you're learning, check out Safari Books Online. They offer a free 10 day trial, that allows you to browse the pages of many great software titles online, so you can find the book that's right for you.
I'm partial to Mordy Golding's Real World Adobe Illustrator series, and I have always loved the Visual Quickstart guides, I just bought the latest one for Photoshop. If you haven't yet upgraded to CS5, don't despair, there are plenty of titles you can browse for the earlier versions of Illustrator too. Happy shopping!