I was fortunate to be among those that purchased the book called
"Im Aufnahmesalon Hupfeld".  It's full of high resolution photos of
Hupfeld artists, but unfortunately for me, it's written in German.
However, it came with a translation, which I notice is now being
offered for sale at $10.

I think this is fantastic value for such a well researched and well
written publication.  It has 28 full size pages of information that
I have not seen otherwise published.  In fact, I find I refer to this
translation far more than the book of photos, which while fascinating,
don't tell me nearly as much as the text.  Highly recommended reading
for all reproducing piano enthusiasts, while stocks last it seems.

Peter Phillips
Sydney
http://www.petersmidi.com/