Hi,
Don't forget Citrix's website, all the documentation is freely available in PDF format and if you can read all these understand them, you are well on your way to passing the CCA exam!!!!
Having a knowledge of the Registry, looking at how programs run and what makes them tick really does help, most programmers don't write programs that are Citrix friendly and your skills would be well used! Saying that, most large software houses are getting better!
Good luck,
Carl.