Alistair Cooke and Eric Fitzpatrick have a book, Helicopter Test and Evaluation. I haven't read it yet, but much of the content is on the course at ETPS. Eric is a flying tutor at ETPS. He knows his stuff and expresses himself quite clearly so I'd expect the book would be good.
There is also a book on homebuilt flight test. Can't remember the author, but the sections I read very clearly laid it in plain language what we've confused with aeroderivatives. It was fixed wing specific, but understanding that will be a good background.