Cornwall studies library (in Druth) has a plan in their plans drawer and the usual compliment of books, as well as a couple of architectural reports about some shaft pluggings.
The CRO has a searchable catalogue here:-
http://crocat.cornwall.gov.uk/DServe/searchpage.htm Off the top of my head, I think there is a pic in the back of "The Central District Vol 1" Trounson/Bullen and another in "Cornwall's Central Mines" by Morrison (in one of the vols). Hamilton Jenkin, Mines and Miners Vol 3 "around Redruth" has some stuff as well.
I think going into the studies library and having a look at those books is probably your best bet.
The bookshop under the railway viaduct in Redruth has a good mining section and is pretty cheap. The Morrison books are essential to anyone really interested in the Central District.