This is one of my favourite books and I've read it twice.
it appears to have been put together from his writings after the author's untimely death and there are chapters by others included (can't remember her name just now).
This is essentially about the Germanic dialect of the eastern part of Lorraine in France, bordering Luxemburg and Germany, which is the same language spoken in Luxemburg. The author goes into the history of the area, both linguistic and political. It is fascinating. There is also some telling material from his upbringing (I recall in particular the moment he and his brother rejected their native language) and even some humour (a funny story about a 100th birthday celebration if memory serves). There's also a good bit about issues of identity. Apart from some of the political history sections (relating to the status of Luxembourg about 200 years ago or so) I found the book to be very absorbing. Finally there is a brilliant graphic/ tabular analysis of modern Letzerbergesch (however that's spelt) within the borders of Luxemburg, in terms of dominant/ dominated status within the various situations where it is used, broadly under the headings of spoken and written. A real eye-opener. It's really for the interested layman as far as I can see and the French is straight forward. Ever since reading it I've had a fancy to visit the area in question.