The book starts off with a gentle introduction into Drupal modules, Drupal themes and jQuery. These first chapters are pretty boring if you are already familiar with Drupal development but they aid newcomers in getting up to speed.
In an earlier post I explained how I used the BlockUI jQuery plugin to prevent multiple form submissions because I couldn't get it to work by simply setting the disabled attribute of the submit buttons.
Some users confuse the concept of clicking and double clicking. This can have nasty consequences when they submit a form and fire off an expensive server job twice because they double clicked on the submit button of a web form.