I've read "Oracle APEX Best Practices" and I’m still surprised, why it hadn't been written earlier?!
The book covers in 6 chapters very powerful knowledge about developing applications using APEX in correct way. Starting with modeling of application and database, through development and deployment tips, ending with great advices about security and maintain.
If you have been using Oracle Application Express for more than several months, you should read this book and check if you use APEX in the proper way. If you are new to APEX, start with book for beginners, but keep in mind you have to learn best practices some day.