For the past months I have been working together with Marcel van der Plas on a book on APEX 4.0. The publisher Packt has just released the website, so it can now be pre-ordered.
Find the link here: Oracle APEX 4.0 Cookbook
The book offers more than 70 recipes that cover almost all topics of Application Express 4.0.
We are currently finishing up and expect that the book will come out in January.