I just put up a page on my blog listing research resources for Philadelphia, including a ton of links to online Philadelphia Directories.
Something I thought y'all might find interesting is a diagram of the Esterbrook factory in Camden, NJ from 1885. It's from a Sanborn map used by insurance companies.
The red buildings are made of brick, and the yellow ones are wooden-framed.
The location is currently a parking lot across Cooper St. from Camden City School District Office. It was in the middle of the block, along Cooper, between Front and Delaware, on the south side of the street.