I achieved the certification of web components with 89%.
It took me about 4 months .
I red Head First JSPServlet of Kathy Sierra and Bert Bates : Amazing, really useful, I think it's the best book for this certification. What a pity it's only for Java EE 5.
Further, I red the book from Charles Lyons for Java EE 6, but I didn't like the structure and approach of the book.
And I red the Servlet 3.0 Specification that's great. I practise a lot on my own website, the professionnal project of my job and a lot in Enthuware.