I read the legendary Monkhouse&Camerlengo book, in addition to this "Head first Design Patterns" by Freeman and a SCJD book provided by Sun (The Broker Tool application). In my opinion the "Broker Tool application" suited the best because it deals intensively with special design patterns being very useful for the assignment and not covered by Monkhouse.
I think this certification exists mainly to prove one's knowledge of using accepted design patterns.
My trainer from Sun Microsystems also says this exam is actually about design patterns.
Of course anybody can accomplish the SCJD assignment without knowing anything about patterns,
but that's surely not the way it is supposed to be.
Since design patterns have little to do with architecture, SCJD is no requirement and only a little help for other exams (like SCEA).
Fire me boy! Cool, soothing, shameless self promotion:
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop