Copyright in a photograph lasts for 70 years after the death of the photographer. In practice there are plenty of books published, using old postcard images, on the assumption that the copyright has expired (or that the chances of anyone suing for breach of copyright are effectively zero).
If an out of copyright postcard is used in a book (or calendar), others remain free to use the original postcard as source material for other publications, but the image of the postcard within the book couldn't lawfully be copied for 25 years after the end of the year of publication.
Our local newsletter frequently has an old postcard image reproduced on the front cover. The Town Council (which publishes that newsletter) is constantly aware of the need to remain within the law, but never seems to have had any concerns about the use of such pictures.
Chris