There is very little documentation of sys.preview. You have probably seen
http://quickstarts.asp.net/Futures/ajax/default.aspx
which is very minimal and probably wont give you enough examples of things you are interested in.
You should be able to put
/// <reference name="PreviewScript.debug.js" assembly="Microsoft.Web.Preview" />
at the top of your js file and get some intellisense support. You can also open up the actual PreviewScript file and look through it. Looking at the descriptors of each control/component and their base components may be the best way to learn about how they are meant to be used.
What will be rough is figuring out the xml script. Outside of the quickstart there is very little anywhere about it. However most everything that can be done in XML could be done in javascript, with a few exceptions, listViews (datagrid/gridview) are extremely difficult to replicate in javascript. But once you get the hang of it you should be ok without any more documentation than just the previewscript.js source file.