The ARM is fairly in-depth, yes - but it's by no means the primary basis for the course material.
- You will get given two more documents when you arrive, which will be your bread & butter
- There is a lot of reference to the AIP so it would help if you were familiar with navigating your way around it
- There is a lot of extra course material presented to you in class, which most of us take notes on
The ARM is handy for the subjects not specific to ATC, ie Principles of Flight, Nav Aids, Met etc - more your generic aviation stuff.
It's been mentioned before but I'll mentiond it again - it is an INTENSIVE training pace - so make sure you have all loose ends tied up as you'll need to give it everything you have. And then some
Having said that - I'm loving every minute of it
