There, on the underside, one friend wrote to another "Friend" (which is what my best friend and I call each other) in Bloomsburg, Pennsylvania (where my parents live now and where I lived since I was a senior in high school), some time in 1911.
What a coincidence!
In what appears to be awkward adolescent script, B.S.N.S. wrote to Miss Naomi Shoop:
You must excuse me for waiting so long to answer your card that you sent me. I was very glad to hear from you we are all well hoping these few lines will find you all the same. Well I guess you are counting the days by this time for you vacation. Jay told me that she can hardly wait till you come home. [illegible]...B.S.N.S.
Before I started writing this post, I conducted a search using Ancestry Library edition (a provenance researcher's best friend!), expecting to find a Naomi Shoop in Bloomsburg (I did not). But once I transcribed the postcard message, I realized that Naomi is probably from the York area. Why else might she appreciate a postcard view from the Market St. Bridge, looking south, in York? (I suppose I am also assuming the sender is from that region.) There were a few Naomi Shoops living in that region around 1910, so it is difficult to say who the original recipient of this postcard was.
Needless to say, she had a good Friend.
For quick primers on the history of postcards, see this resource at the Smithsonian and this one from the Metropolitan Postcard Club of New York City. Chris Otto at Papergreat writes about postcards often. Check out his blog for unusual postcard specimens. Also, The Ephemera Society of America, the leading organization of ephemera enthusiasts, includes resources associated with postcards on its website. For any ephemera research, start with Maurice Rickards' Encyclopedia of Ephemera: A Guide to the Fragmentary Documents of Everyday Life for the Collector, Curator, and Historian.
Aside from the popular "collector's" literature, not much has been written on postcards. If you're interested in more scholarly perspectives of postcards, you may want to take a look at Bernadette A. Lear's "Wishing They Were There: Old Postcards and Library History," Libraries & the Cultural Record 43, 1 (2008): 77-101. In this essay, Lear writes about her own collection of postcards that depict libraries and how we might use those postcards to learn more about the history of libraries. It includes a good overview of the history of postcard consumption at the beginning. Winterthur Portfolio published a fascinating article on an early twentieth-century cache of paper ephemera and other artifacts one professor of urban studies found in his home in St. Louis recently. Joseph Heathcott, "Reading the Accidental Archive: Architecture, Ephemera, and Landscape as Evidence of an Urban Public Culture," Winterthur Portfolio, 41, 4 (Winter 2007): 239-268. Heathcott does not focus exclusively on postcards, but they comprise part of the collection he interpreted in his article about class, upward mobility, and modernity. Finally, postcards also served more macabre ends in history. For example, sending postcards depicting lynchings of African Americans was popular during the early twentieth century. To learn more about this phenomenon, see the digital collection Without Sanctuary: Photographs and Postcards of Lynching in America.
I'd love to hear about other postcard resources from my readers!