As some other reviews point out, this book won't teach you much about JavaScript per se, but it will teach you an awful lot about the JavaScript library, JQuery. At first, I was put off by this because I was convinced I needed to learn "raw" JavaScript. This book changed my mind.
"Raw" JavaScript can be quite cumbersome to use and many frequent tasks facing a web developer have to be written from scratch and tested for compatibility across multiple browser versions. Rather than attempt all this yourself, the author advises you to use a JavaScript library (e.g. JQuery) where many of the repetitive tasks facing web developers, e.g. building galleries, validating forms, hiding/showing elements etc. have already been written and tested for you. JQuery is basically JavaScript with most of the cross-browser incompatibilities eliminated and with added, powerful functionality to handle the most common tasks facing web developers.
Having seen the power and convenience of JQuery, I wouldn't worry too much about learning raw JavaScript - unless you had very specific requirements that JQuery couldn't handle. Indeed, this book has made my other purchase - a dry 800 page book about JavaScript - obsolete.
The best thing is that it tackles all the most common tasks you'll face in the "real world" - just take a look at its contents to see what I mean. Indeed, I can't envision a client-side feature that you couldn't implement with JQuery - and this book shows you most of them.
My only gripe is that there is a lot of repetition in the book. Some may welcome this as it certainly does make the text easy to read and understand but it also adds to its thickness :) Personally, I think by the time you get to page 260, you should know that line X is creating a variable or line Y is calling a function etc.
Thoroughly recommended for those who want to add a bit of JavaScript spice to their websites without taking the masochistic route of learning raw JavaScript. An understanding of HTML and CSS is recommended.