When I took the SCJP, I used a couple of study guides specifically written for the exam, and found them useful (especially Mughal & Rasmussen). I was wondering if someone has published one for the SCWCD?
No one has published one yet. There's a list of resources, such as specs, servlet and JSP books, that can be very useful for preparing for the exam. There's also notes from people (like Mifta Khan) outlining information for each section. You can find those notes under www.javaranch.com/scwcdlinks.jsp. However, you should not base all of your studying on just these notes. What I have been doing is taking those notes as a guideline and adding additional information to clarify points within these notes - sort of my own certification book.