I know people laugh when they see these titles for IT books, but this is a good one to get you the start you need to enter the world of ethical hacking.  It will give you the foundation for understanding security and is a good tool for such exams.  Don should have a good link to get this for a good deal and hopefully can pull off getting a sample chapter.  It is very well written and goes over the rudiments of hacking.  Buy this if you are curious about hacking or are going for a cert such as CEH.  It helped me when I started!  

D'oh!  I am working another crazy shift and am caffeined up!