SCWCD6 exam is different from SCWCD 5 exam. the SCWCD 5 exam is based on servlet spec 2.4 whereas the new exam is based on servlet 3.0 specs. there have been addition of new objectives to SCWCD 6 exam like asynchronous servlets, annotations, security , programmatically adding servlets, filters etc. head first servlet and jsp 2nd edition book is for old exam. it does not target new exam. you have still a weeks's time left. i recommend you to read servlet 3.0 specs focussing on asynchronous proocessing, security and annotatinos and pluggability chapters. if however you are giving scwcd 5 exam then you are fine with just hfsj.
for the complete list of objectives you can check official oracle site.
all the best