Not a free book, but Barnes and Noble has a set of 13 Alice in Wonderland character sketches by Tim Burton available for free on their website. They're advertised as nook screensavers, but anyone can download the zip file. The images are in *.png format. Look on the right hand side of the page for the link to download the zip file.
http://www.barnesandnoble.com/alice/index.asp?
Here's an example of what they look like.