I just received my copy of Expert Oracle Application Express last night, and I have to say–it's definitely worth the money. I've only read one chapter so far, but there were many points in the chapter that I simply hadn't found out on my own before. This is going to have an impact (purely positive, of course) on how I build tools going forward, and I'm very excited to read more.
The structure of the book is slightly different from other technical manuals I've seen, largely as a result of how it was written. Generally, technical manuals start with an overview and then drill down into a tool, so reading early chapters helps understand the later chapters. Expert Oracle APEX was written in a more modular form–there were thirteen authors, each of whom wrote one chapter. So you can read each chapter independent of the others. These thirteen are all people whom I'd consider masters of APEX, and reading the book is a great way to learn their best tricks.
As the title indicates, it's not a book for beginners, but if you're moderately comfortable working with APEX, you'll gain a lot by adding this to your shelf.