Ext JS framework has enormous amount of functionality therefore most beginners would be overwhelmed with that. Although Sencha offers various learning material, learning Ext JS is still a difficult task.
"Learning Ext JS" is a light in the dark for many beginners who want to get an overview of this huge framework.
This book introduces many aspects of the Ext JS, sometimes concisely, and sometimes in detail. Especially, the description of fundamentals (class system, component lifecycle, data packages, etc) is well-organized. I can assure you that you will begin with Ext JS smoothly.
Plus, many supplement tips is embedded in a lot of places. It would be useful for not only beginners but also seasoned Ext JS developers.