I have learned basics of ExtJS and developed some web apps using ExtJS. Now I have to develop web desktop using ExtJS (like desktop app which is present in examples folder) but I am not able to find any documentation or tutorial or book about how to develop it.
Does anyone knows how to develop web desktop application using ExtJS 4? Where can I get any tutorial/ book/ video about developing web desktop?
I doubt you'll find a book or tutorial dedicated to exactly what you're looking for, but you can put together enough resources across the internet to do the trick.
A Google search of "extjs 4 cookbook" turned up a book called "Ext JS 4 Web Application Development Cookbook" that seems to have plenty of information. However, it seems very new and I personally haven't read it, so I can't vouch for it. But it's there.
Other than that, Sencha's own documentation site will have most of what you need. The series of articles on App Architecture may be of some use if you're planning to take the MVC approach, as will the articles on components and layouts.
That will cover the basics, but there's no real definitive guide to making a web desktop app that I'm aware of. It all depends on what your requirements are, how much time you have available, etc. If you're looking for help with a specific component (like creating a Windows-style file browser system) then you'll probably have better luck asking more narrow questions.