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.

=========================================================
Important Subscriber Information:

The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . The body of the message should read "help" (without the quotes).

If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).