Best book is Data Services Technical Manual + Your practical knowledge on the subject area.
I have seen some questions coming up based on a scenario. So to answer them, atleast I believe we need a practical knowledge.
Hope you have a SAP ID first of all? If not follow this thread:
And the below thread about the Exam and link to the Exam Details on SAP Page
ganeshxp 
(BOB member since 2008-07-17)