Hi Alice,
now why would anybody write a book first of all only about a 25 year
history and second in English about a state in Germany. Can you
imagine a similar history of New York state written in Bulgarian or
German? 
As it is nothing like that would even exist in German. Histories are
written about geo-political places in epochs or large time frames if
not in total.
You can imagine that Oldenburg was a duchy ruled by a grandduke until
the end of WWI. It then became a state like any other and very similar
to states in the US. Life in the industrialized world in the early
20th century was quite similar.
Fred