There was a very good book out in the 1980s regarding the history of postcards. Actually, I think it was about the history of photographic postcards.  I used it and some other resources to do a paper for the History of Photography class.  Your local library should be able to help you locate a copy of this publication.  I'd give you a more exact citation except my information is packed in some box at home and I am at work.
The title might have been as direct as History of the Photographic Postcard - or that may have been in the subtitle.


 
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