I'm interested in learning about the history of making potpourri. Can you direct me to some reference books or other reference sources?
Good question -- there's probably a PhD. thesis or two lurking there. I haven't seen anything that I'd really call "history" rather than "lore" about the origins and development of potpourri. Here's some guesswork:
It seems certain that the first stuff to be called potpourri was made by the "wet" method, in which rose petals were not completely dried to a crisp before being layered with salt. It's basically the same method as making sauerkraut, a process of fermentation -- and the word potpourri means "rotten pot" in French. I've read that jars of rose petals have been excavated from ancient sites in Egypt, so perhaps potpourri actually goes back that far. Potpourri jars -- opaque, with a solid cover to keep scent in and a pierced cover to let it out -- date back a few hundred years, I think, but I have no idea where and when the first one was made. "Strewing herbs" were used in the middle ages to sweeten the air, but I don't think they really count.
Dry potpourri is usually fortified with essential oils, and essential oils don't go back further than steam distillation, which is late middle ages. I suspect that growing flowers in the quantities needed for essential oil production is a product of the industrial revolution, with railroad transport and the prospect of using farmland for growing specialty cash crops.