Hi Maria,
1. No, the book assumes you have a basic grasp of Javascript and client-side developmnet in general (i.e., HTML, CSS, DOM, etc.) but not AJAX. If you have never looked at AJAX before, but know Javascript a little, you'd be able to jump right in.
2. That being said, if your specifically interested in AJAX, especially since you have a
Java background, my first book, Practical Ajax Projects with Java Technology is almost certainly the better choice. Ajax is only one part of the latest book, whereas its the focus in the first book. I go into much greater detail in the first book, show a wider range of usages, etc. However, you will find that the last chapter in the current book looks a great deal like the first (or maybe second, I forget) chapter of the first book (and you'll also quickly realize how authors who write more than one book on similar topics can usually get subsequent books done a little faster LOL)... it's an introduction to AJAX, and may well be all you really need because frankly AJAX at its core is pretty simple (although there's certainly higher level concerns that make it less so).
Bottom line, after all my babbling: the Practical Ajax Projects is likely the better choice for you, but I'd say take a peek at the TOCs, and also the sample chapters around, and see what you think (FYI, I'd say maybe a full half of the first book is actually available online in various places!)
