Stryda,
Firstly, good luck with the RAF career. I assume you have done your research and know what the role of the AMM is? Have you managed to speak to anyone who is actually doing the job now?
As for free material to help with your studies, not sure really but you can always spend some time on google searches and see what comes up! or try your local library. Some librarys have online search facilities, so you can even look without leaving the house.
Again, good luck and when you get to your first squadron ask the lads if you can play 'spoons' at your first beer call.
You'll have a cracking time.
Chad