Wikipedia's own Phoebe Ayers, Charles Matthews, and Ben Yates have written and published a book about Wikipedia. The book, How Wikipedia Works, is published by No Starch Press and is released under the GFDL license. Yes, that's right: a printed book released under the GFDL.
Is book. Is Wiki. Is GFDL. Is Wikibook? Not yet, but I wouldn't be surprised to see it get translated to wikitext in the future. After all, the beauty of wiki-based textbooks is that they never need to get out of date, and they can be updated as things change by anybody who is interested.
By the way, 5% of the proceeds go directly to the WMF, so that should make you feel better about it.