A question which gets asked a lot round these parts is: "Do I have to read the spec. because I can't seem to understand it".
Personally, I think the specs. aren't all that hard to read - and it is an important skill to be able to read such documents. But their primary use is to act as a reference for container and application developers, to be the definitive guide when things just don't seem to make sense or where confusion can otherwise arise.
The specs. will not really help you understandJ2EE Web, and they won't really help you to revise for the SCWCD either... They are not written as either tutorials or as a condensed reference. There's lots of quirky things in the spec. which perhaps you should be aware of, but never appear in the exam!
So no, you don't need to read the Servlet spec. (or JSP spec for that matter, which is 478 pages) to become a good Web developer or to pass the SCWCD.
I can't comment on Head First because I've never read it. But if you're interested in knowing more about my book, which is a more traditional text but with excellent explanations, diagrams and good reference material ( ), over 300 SCWCD questions, plus a couple of additional chapters on the new Java EE 5.0 stuff, please sign up to my mailing list (the link is in my signature) and I'll send out loads of details (TOCs, sample chapters etc.) in a couple of weeks.
Charles Lyons (SCJP 1.4, April 2003; SCJP 5, Dec 2006; SCWCD 1.4b, April 2004)
Author of OCEJWCD Study Companion for Oracle Exam 1Z0-899 (ISBN 0955160340 / AmazonAmazon UK )