Obviously, Practical Prototype and script.aculo.us covers all you need to know about the latest versions of Prototype and script.aculo.us. But it goes well beyond that. Andrew does an awesome job at setting the context and giving appropriate background information, so much so that you'll end up knowing not only the how but also the why. In the ruthless world of client-side development, that's a serious asset!
Practical Prototype and script.aculo.us is a pleasure to read -- the style is both straightforward and witty -- and should appeal to beginners and seasoned developers alike.
If you want to try before you buy, you can always download a sample chapter or the table of contents from the Apress website. Or you can grab a hard copy and/or a pdf from the Apress website or from Amazon.
As always, happy Prototyping!