What may work for one person may not necessarily work for you. However, if you feel that you need coaching then they will recommend what they think works best.
Whatever you choose to study with when it comes to the exam take the time to read the question carefully. That is not a quick scan and then select an answer but really read the question, more than once. In many ways I think IREX is the most difficult of the CASA exams as it drills deep into the subject and you need to know exactly what is being asked to be able to answer the questions.