I won't pretend to be an expert on ExtJS - I'm not. But I do know a number of customers who use Oracle Application Express combined with ExtJS and they love it. Additionally, I do know some of them have informally consulted with Mark Lancaster, as he is one of the recognized experts on the planet for Oracle Application Express and ExtJS. Mark's Playpen is his hosted demonstration site of the types of things you can accomplish with APEX and ExtJS.
I've browsed through this book and it seems to be another home run from Packt - very clearly presented and very polished.