What sort of reference material?
If you are training to be a fixed-wing pilot, is this PPL or CPL. Either way, it hardly matter since much study material is common.
At PPL level the Air Law exam has a helicopter slant but the information is still all in Trevor Thom. You need Principles of Flight book (Wagtendonk is best IMHO). ...er... that's it!
Are you after stuff that is free on the internet? Can't think of much specific to helicopters which is useful for training that you wouldn't already have as a plank pilot.
There are very few helicopters books that I don't have so if you want to know any further recommendations, please ask.
Cheers
Whirlygig