Comet is a long-held HTTP request allows a web server to push data to a browser, without the browser explicitly requesting it.
I was a little bit surprised that the book only gave two pages to CSS Sprites. My testing shows that sprites can significantly improve web site load times if the site has multiple small images coming from the server.
Web app efficiently is going to start playing a more and more significant role as we return to the mainframe model with cloud computing. Many of the cloud computing providers are charging by processor cycles so the fewer of these your app uses the less expensive it is to run in the cloud. On top of that because you are processing your requests faster your site will be more competitive.