"Reichsdeutsche" or "Imperial Germans" is a term that came into being after the formation of the German Empire in 1871. Its main usage was to properly distinct between being "German" - back then, German-speaking citizens of the Austro-Hungarian Empire as well as ethnic Germans in Russian territory or even the US would self-identify as "German" too. Being "Reichsdeutsch" therefore simply meant being a German living in the German Empire (Reich). There is an updated version of that, when e.g. Austrians refer to the German spoken in Germany as "Bundesdeutsch" (federal German).

The legal background of being an "Imperial German" is quite complex, however. The German Empire was a federation of various states, and each of which had its own rules of citizenship and how to aquire it. These many different local laws were kept in the books even after 1871. The first German Imperial Law regulating citizenship only came in 1913, over 40 years after unification, and even then it only raised national standards to which state citizenship laws had to adhere. Until 1936, there was in fact no "German" citizenship (besides a small handful of exceptions e.g. for representatives of the Empire who permanently lived abroad); people were carrying a Bavarian, or Prussian, or Saxon etc. citizenship and were Imperial Germans by virtue of that. This was only ended by the Nazis in 1934, when the individual states' citizenships were abolished in favour of a unified German citizenship. The term "Imperial German" had been frequently used for almost a century before that, but only after 1934 there actually was a legal background to it.

"Reichsdeutsche" were German who hailed from the German Reich, in contrast to (self-identified) Germans living in, say, Vienna, or Riga, or Prague. The expansionist politics of the Nazis pre-WW2 and the various annexations during the war meant that the number of "Imperial Germans" quickly expanded as well, at least legally speaking. I'm not sure what definition of "Imperial German" the author of your book actually uses. Interestingly, the city of Auschwitz itself as well as the camps erected in its territory were made part of the German Empire by October 1939, whereas the regions only a few kilometres to the east belonged to the "General Government", a zone of occupation whose legal status was never fully clarified and which often was described by Nazi officials as a Reichsnebenland or "Country Adjacent/Subordinate to the Empire".

All of this was to say what "Imperial Germans" actually were both as a legal term and in everyday language. I am, however, not aware of any large-scale resettlement of Imperial Germans into newly occupied towns. There were almost a million ethnic Germans mostly from Eastern Europe who were ordered to move into those parts of the Empire that had recently been annexed, mostly the Wartheland area to its very east. Those Germans were very much not "Imperial Germans", however! They were "Volksdeutsche" or ethnic Germans, who had previously lived in areas like the Baltics, South Tyrol, Bessarabia, Galicia or Transylvania, i.e. outside of the Empire's boundaries both before and during the war. They were not moved into merely "occupied" areas either, but in those like the Wartheland which, according to the Nazis, were fully part of the German Empire now. I don't know about any relocations into occupied territories like the General Government.