A book specifically about creating Demonstrations? No, I know of no such books.
More generally, about getting started with Mathematica? Yes!
Actually, I recommend beginning with one of the introductory videos on the Wolfram web site, and most especially those by Cliff Hastings or Jon McLoone, e.g.:
http://www.wolfram.com/broadcast/video.php?c=86&v=357
http://www.wolfram.com/broadcast/video.php?c=86&v=906
http://www.wolfram.com/broadcast/video.php?c=86&p=2&v=327
A good beginner's book is "Hands-On Start to Wolfram Mathematica", by Cliff Hastings, Kelvin Mischo, and Michael Monson, which is available in paperback.
For more suggestions, go to the Mathematica Learning Center (https://www.wolfram.com/support/learn/) and look at the list of resources at: http://mathematica.stackexchange.com/questions/18/where-can-i-find-examples-of-good-mathematica-programming-practice/259#259
Finally, don't forget the incredible resource provided by the Documentation Center that constitutes the Mathematical help system.