Programming for Dummies: http://www.dummies.com/store/Computers-Internet/Programming....
Sams Teach Yourself X in 21 Days: http://www.informit.com/search/index.aspx?query=21+days
The Complete Idiot's Guide to X: e.g. http://www.help4web.net/webmaster/Java/NewJS/JavaScriptIdiot...
Maybe those series are no longer as well known as they used to be? They were household names in the '90s and everyone would've gotten the references, but it's possible they're now dated and better removed from the intro.
The draft version (linked by edmccard in another comment) now has a brief explanation in the margin note that these are references to programming books: http://www.ccs.neu.edu/home/matthias/HtDP2e/Draft/index.html
The draft version (linked by edmccard in another comment) now has a brief explanation in the margin note that these are references to programming books: http://www.ccs.neu.edu/home/matthias/HtDP2e/Draft/index.html