If my memory serves me right the RNAV extras came in as part of the JAR syllabus. I used a book which is called "JAR Supplement" to the Jeremy M Pratt series of books. I think it is now listed as Exercise 18C and encompasses VDF, ADF, VOR and DME.
I am just flicking through the chapter and it seems to have all the relevant info, I would say is a good place to start
Gertrude
