This gives a pretty good introduction at what is actually going on. The framework I've used (Prototype) abstracts away the details (like constructing the XMLHttpRequest) but its always good to know whats going on below the surface.
If you know any python, this link - http://www.willarson.com/blog/?p=36 - is very helpful, it was how I got started. The actual AJAX part of it is the same no matter which server side language you use.
Prototype is really a horrible framework. It works by adding methods to every class in the language, even when they don't make sense. This makes for a bit of syntactic sugar, but it also ensures that you can't use other frameworks and that custom code might become very hard to debug.
Layering frameworks probably isn't a good idea to begin with. Prototype does seem, unfortunately, more powerful than jQuery, though, when it comes to class creation.
2) Professional JavaScript For Web Developers - Nicholas Zakas
I would recommend these two books to anyone who wants to learn JavaScript and XHR. Ajax is very small part fo the whole picture if you are totally new to javascript
This gives a pretty good introduction at what is actually going on. The framework I've used (Prototype) abstracts away the details (like constructing the XMLHttpRequest) but its always good to know whats going on below the surface.
If you know any python, this link - http://www.willarson.com/blog/?p=36 - is very helpful, it was how I got started. The actual AJAX part of it is the same no matter which server side language you use.