In addition to the already listed sources on gravestones you might consult:
 
Bell, Edward L.
    1994    Vestiges of Mortality and Remembrance: A Bibliography on the
            Historical Archaeology of Cemeteries.  Scarecrow Press, Metuchen,
            NJ.  (Pardon the pun, but this is literally the last work on most
            cemetery subjects).
 
Gillon, Edward V.
    1972    Victorian Cemetery Art.  Dover, New York.
 
The publications of the Association for Gravestone Studies, particularly
their yearly journal Markers.
 
A publications list is available from:
 
AGS
278 Main Street, Suite 207
Greenfield, MA
01301
 
Good luck!
 
Rich Veit