Sridhar explains the concepts of Script.aculo.us, Prototype, and AJAX in PHP. The chapters are well-written and follow up with many useful examples. The author weaves those concepts into reality and explains how many of the effects that we enjoy on the web are done. My favorite was creating a Delicio.us / Digg -like bookmark application along with many effects such as search with real-time autocompletion, realtime updates, and tag clouds. Another useful chapter describes an AJAX enabled shopping cart that provides awesome features like drag and drop, and applies some of the interesting Script.aculo.us effects to the online store. Very cool.
The author also writes in a fun tone, and gives the reader a lot of interesting tidbits to chew on. A good read overall.