They present a problem and a solution in the form of quick articles and they are usually based on common issues for that technology.
This book in the series presents 80 different hacks or techniques to create AJAX driven web sites. From simple stuff like a basic cross browser XHR library to more complex stuff like mash-ups, use of Prototype, Rico and Direct Remoting.
The code examples are good but I found that some of them are not really ready to copy and paste into your application and you should look into them as starting points.
For example in some demos using Prototype the make use of both the
$() function and the DOM equivalent
document.getElementById(), I could like to see some consistency in there.
Besides that, the book is really good and you will find yourself flipping through its pages while putting together your next AJAX application.