The book used to be essential reading to get past stage 3 interviews, but stage 3 is entirely competency based now so you don't need to know anything about ATC.
It may help in general with stage 1 and stage 2, and will certainly give you a better depth of knowledge, which is extremely helpful when you tour units as you know what you are doing, and come to realise whether or not being an ATC is a job you REALLY want or not. Also very good to have a general knowledge of the job when you talk to other ATCOs as you never know when one of them might be interviewing you or training you!