How much of an understanding do they have of the OF Mass? We can argue as long as we want but the EF is not such a departure that you need anything beside a missal to understand the basic differences.
If they want to start from the basics I would go with the book about the EF by Maria Montessori “The Mass Explained To Children”. If they really want to have a better insight of the different aspects of the Mass and ways to look at it with a special reference to the EF I would suggest “The spirit of the liturgy” by Romano Guardini. If you want a textbook then you have to go with the two volumes of “The Mass Of The Roman Rite : Its Origins and Development” by Joseph A. Jungmann.
BTW do not forget “The spirit of the liturgy” by our Pope.
If you search CAF there are also a couple of other books that I do not have and so I cannot speak to their quality, however, my impression is that they are really worth reading.
