From the Database-Geek.

In addition to frequently being asked to recommend books for newbies to Oracle, I am also asked for guidance on certification. Basically, what materials are available and worth purchasing. I used Self Test software and the Sybex books when I was working on my certification. That was back in the 9i days. If you are working on 9i certification, I strongly recommend them.

However, I figure you are probably working on an 11g certification; 10g at least. In that case, I would guess that Sybex and Self Test are still good options, I just haven’t personally worked with the latest versions (software or books).