I read this book having no knowledge of Javascript or Ajax to start with, and found it very clear, helpful and comprehensive. There are lots of examples, and the authors take the reader through things step by step. Towards the end there are sections for specific actions, such as creating rollover buttons and drop down menus, and if you want to you can just type in the scripts to get your page to work.
You would need a basic understanding of HTML and CSS and so on to fully understand this book; they do take you through those in sidebars, but I'd say this mightn't be enough if you really knew nothing about either - the authors themselves say that they expect readers to have some familiarity with basic HTML and CSS.
The book is accompanied by a website which allows you to download the scripts, gives examples, and gives access to downloads of chapters which appeared in earlier editions but have now been removed.
If you've never done any programming before, the book is a good starting point for that; things are introduced slowly and with lots of explanation.
The style is casual and chatty, and the book is enjoyable to read.
It's probably not enough for advanced javascript, but that's not the aim and there are plenty of other books which give more advanced techniques and which would be useful as follow-ons from this one.