The reference books won't help you decide what to study, it would be great if they had little Breadth or Depth indicators in the section headers... but they don't. So it is upon you to decipher what to study.
Several times now, I have realized far too late that the topic I am studying in the CERM or AIO is not relevant to what I need. In most cases this is due to me stumbling into a section of a chapter that is for the depth test on the topic instead of the breadth.
Just today I was studying the Hydraulic Machines chapter of the CERM (Chapter 18). This section covers pumps and other hydraulic machines and things like efficiency, speeds, horsepower, changes in energy head etc. All of those things will be in the breadth section so it is good I studied them. However after covering speed, the chapter jumps straight into cavitation and suction which only appear on the hydraulic depth section. I will admit those things are interesting but I probably should been studying something that will be relevant to me.
The easiest way to stay on course is to constantly refer to the exam specifications on what to study: