Welcome to the CPL Navigation question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
That is really up to you, the workbook is a exercise book as in lots of repetition of exercises to help make you proficient, The exam preps are different as it's trying to give you a feel of how the CASA exams are presented. The exam preps would be my suggestion along with the workbook
but it depends on how you confident you feel your knowledge of the subject is.