Quick disclaimer: I received a gratis review copy of this book in exchange for writing about it.
With Ext JS 4 bringing so many changes to the framework, even experienced Sencha developers need a helping hand to understand this new direction. Ext JS 4 First Look hits the sweet spot for both new and existing developers. It doesn't go in-depth, but that's why it's a "first look"; the book touches on all parts of the new release and gives an excellent overview of changes and new features.
The chapters on theming and MVC are particularly welcome, as these are both important facets of Ext JS 4 but ones which are still not well documented. In particular the brief practical examples for theming components and creating custom UIs are a good introduction.
Overall Ext JS 4 First Look serves its purpose well - developers who are new to Sencha will get a feel for what Ext JS is all about, and existing users will see how things have changed since version 3. Kudos to Loiane on this great release!