Editor: So, the book has been describing how to be a practised user of the volume control.  Now, from Dad’s text:

One curioius effect may be noticed at this stage.  The loudness of background noises relative to each other is enhanced, as if the scale of loudness of noises has been stretched out.  This is exactly what happens with a hearing aid of course, but you will be protected from the effects of really loud noises, as we later describe.

Editor;  I could write a text book on that paragraph.  Dad was aware that his ears had abnormal loudness function, and that the hearing aid had to somehow compensate for this, and he accepted the limitations of how it was done.  Dad experienced  the hearing aid history of linear amplification , with various limiting processes, and then at the time of writing this he would have been wearing a fairly basic compression hearing aid (remember, it was written about 13 years ago).  Different academics and hence also different hearing aid companies, support different academic theories of the most suitable way to compensate for abnormal loudness growth.   Compression schemes are all a lot more sophisticated now, than they were when this was written.    With the ADRO processing in the Blamey & Saunders Hearing aids, the whole topic is circumvented, by putting the signal into the listeners usable range of hearing.