Do you know which study route you’re going down yet in respect of Aerodrome or Area?
I’d suggest the air traffic control handbook by David Smith. Don’t bog yourself down too much with the CAP publications although there is a lot of useful info in MATS pt1 but by the end of the course you’ll be regurgitating it word for word anyway. If you’re going into Aerodrome perhaps flick through section 2 of the 493 but don’t get hung up on anything you can’t get your head around due to the way it is written in parts