The SCJA is not a pre-requisite for the SCJP exam, but it is a great place to start your journey into the Java certification world.
From what I understand, SJCA, SCJP and SCEA are the three Sun certifications that do not require and prior exams to have been passed in order to take them.
I'm the author (Cameron McKenzie) of both the SCJA Certification Guide and the Exam Questions Guide (which has over 350 SJCA based mock exams), so I'd obviously recommend those two books. A good friend of mine is coming out with an alternate SCJA guide later on this year. My website, www.scja.com, has some mock exam questions as well that can help give you a feel for the type of questions and level of difficulty of the SCJA exam. Ok, enough plugging myself.
One thing about the SCJA exam is that it is like a river that has overflowed its banks. It's surprisingly wide in it's coverage of topics, but at no point does it get overwhelmingly deep. So, with SCJA, you need to know Java syntax, a little about Servlets and JSPs, some knowledge of micro devices and even EJBs, and a little bit of OOA, OOD & UML as well. Compare that to the SCJP exam, where there are no questions on EJBs, no questions on Servlets or JSPs, no questions on micro-devices, and from what I recall, not testing on UML.
The SCJP exam is really just language and syntax. So, you need to know some really bizarre and esoteric facts about Java and the JVM in order to get a high score on the SCJP exam. The SCJA exam is nowhere nearly as intensive when it comes to Java syntax and language constructs, and for that reason, many people find the SCJA exam to be a bit easier to pass.
So, SCJA really proves that a person is a well rounded Java professional. It's a great base for any Java career.
Check out my signature links about the Sun Certified Java Programmer exam for some more info.