While undertaking this endeavor, I am doing my best to see if it is possible to do this using the Oracle Documentation alone. That means no study guides (other than the topics covered), no third-party books, nothing.
I don't believe I have ever purchased a book on or about Oracle. The documentation provided, especially for the database, is top notch. There is so much information available out there if you just know how to find it. Reading AskTom for years didn't hurt either.
Here are the books I have used to study for this exam. I will update this list as appropriate.