I would like to learn to use Mathematica to develop applications such as those presented in WOLFRAM DEMONSTRATION PROJECT.
The problem is that I do not know where to start.
Could you help me ?
Some simple-minded suggestion:
Thanks for your suggestions.
Could you recommend a good book?
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.:
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.
The Mathematica GuideBook by Michael Trott always gives me lots of inspirations. I like exploring the SystemFiles folder. Some interesting tricks are hiding there.
You can also check this by Leonid Shifrin:
"Mathematica programming: an advanced introduction"