Graham Dukes book, A Laymens Guide to ATC, worked for me. I used other material (internet, pprune, visits). I dont know if it is the best book around, but would give you some understanding.
I found the visit to the units the most beneficial, as I could ask ATC scenario type questions to the operational ATCO's, information you cant read in any book.
I basically got my visits , by phoning up the security desk at the units, and after a few diverted calls, being very polite and explaining my circumstances managed to get a visit. You should try your hardest to get a few visits in, even very small non NATS units. It was the first question they asked me at my interview.