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.