Agreeing with CJayne, 'ATC' by Graham Duke (9th Ed) gives quite a good overview of the job.
Some other books which I found out about which may help if you're interested;
'Air Band Radio Guide' also by G.Duke (6th Ed) helps understand how most of the RT take place.
'UK ATC' by David Graves. Even though that thing was written in 1989 and reprinted in 1990..... a lot of the fundamentals are the same. It goes into depth with each one of the London Airports and how their traffic is controlled which is quite cool.
Don't be tempted into buying 'Air Traffic Control Career Prep' by Patrick R.Mattson if you ask me. It's all pretty much strictly directed for AT-SAT (FAA's ATC Selection and Training) and kept VERY americanized as one would imagine... I only found the Flight Progress Strip chapter of any help.