If you use Illustrator CS4 or CS5, don't miss the wonderful "how-to" guides that Adobe has created. These guides consist of original artwork created by Von Glitschka, Greg Geisler, Philippe Intraligi, Shadow Chen and other talented illustrators. Accompanying each piece of artwork is a PDF that documents the steps that the artist took to create the artwork.
The PDFs provide a peek into the technical aspects of how each artist uses Illustrator to execute their unique style. The guides are a good source to get inspired to use some of the newer features of Illustrator CS4 and CS5.
The PDFs are installed into your Adobe Illustrator CS4/5 program folder, in the Cool Extras/en_US/Sample Files/Sample Art/How Did They Do That folder. You can also view a few of the CS5 how-to guides on Adobe's Web site.