Can anyone point me to an article, book, or something that talks about the ideas behind building a basic API for a site? I've made good progress on the various pieces of the puzzle, but I've got an incomplete picture in terms of how things fit together.

This is sort of a continuation of an earlier post about the client-heavy approach that seems to be gaining popularity these days, Which technologies are key in a set up where the client does lots of the heavy lifting and makes calls to an API on the server for content? So far, it looks like...

JavaScript and available libraries can take care of a lot of the client side.

Data can be moved between the client and server via JSON, which is much lighter than XML.

Is it AJAX that one uses the make the call?

I'm trying to get a handle on how all the pieces fit together. Maybe I should dig through the jQuery source and see what getJSON() does?

I honestly don't expect hand-holding and I have done some digging on my own. A bit of a nudge or hint would be very much appreciated.