Mark Pilgrim's book, Dive Into Python, is available as a real book as well as a downloadable (free!) e-book. Interestingly, "the book is published under the GNU Free Documentation License, which explicitly gives anyone and everyone the right to publish it themselves." And sure enough, someone did, and started selling it it on Amazon.com!
If you still don't see the point behind the whole thing -- the GNU Free Documentation License -- you really have to read this post:
So I am grateful for this anonymous soul who woke up one day and said to herself, “You know what I should do today? I should try to sell copies of that Free book that Pilgrim wrote.” Grateful, because it afforded me the opportunity to remind myself why I chose a Free license in the first place. My Zen teacher once told me that, when people try to do you harm, you should thank them for giving you the opportunity to forgive them. In this case it’s even simpler, because there’s nothing to forgive, just explain. She’s redistributing the work that I explicitly made redistributable. She’s kind of the point.
Now go read Cory Doctorow's post on his latest experiment in making his books available for free download.