I don't know about books, but you can follow my signature links for some mock exams.
My portlet development book started as a certification guide for the IBM test, but I decided to take the IBM stuff out and make it a standard JSR168 learning guide. Still, it covers most of the portlet development objectives on the IBM exam.
You showed up just in time for the waffles! And this tiny ad:
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop