I have found the IBM Redbooks to be very helpful in preparing for exams. You can buy them or download PDF version for browsing or printing. The only problem is that there is often too much information.
The best thing to do is look at the Test objectives and read the sections dealing with those topics. The topics may be found in different books, eg. Tuning Guide, Installation Guide, or Programmers Handbook.
Search for Tests by number or certification here.
http://www-1.ibm.com/certify/news/ Search for Redbooks here.
http://www.redbooks.ibm.com/ Practice tests here ( You'll need to create an account, but it is worth it if you are going to take an IBM test) The test engine is done with
Servlets.
http://certify.torolab.ibm.com/icetest.jsp Good Luck to all. I'm taking a test next week myself.