-) Expert Oracle Application Express Plugins by Martin Giffy D'Souza
This book is almost a year old, but it's still a great resource if you want to build Oracle APEX plugins.
Martin explains very well the different plugin types and shows how to create them. I like the way he wrote the book as he also explains the business need for the different plugins.
-) Expert Oracle Application Express by Dietmar Aust, Martin Giffy D'Souza, Doug Gault, Dimitri Gielis, Roel Hartman, Michael Hichwa, Sharon Kennedy, Denes Kubicek, Raj Mattamal, Dan McGhan, Francis Mignault, Anton Nielsen, John Scott
More than a year old, but still a great source as so many different authors wrote about their expertise, so you get a big variety of topics. The royalties of the book also go to a good cause.