I also found the Pooleys Air Pilots Manual on Communications and equivilent exam question book to be pretty good.
I would advise taking formal training from a qualified RT trainer.
For those in the UK, I can personally recommend Malcolm Dobson, based at Leeds Bradford Airport.
Malcolm does 2 day courses each month leading to the exam.
Cost is 180 inc VAT and including the exam fees.
He also does one 2 one training, and is a qualified examiner.
Based Multiflight Training Room, Leeds Bradford Airport.
07708 842143 / 0113 258 6306