Thanks for the article which I find very interesting. As you say the process will have evolved over the years, certainly in my lifetime, up to the early 1970s I cannot remember anyone calling at my home to canvass regarding Directories., then again with both parents working and both kids at school, it must have been difficult for the canvassers to find people at home when they visited.
Regarding the production of old books, there is a link below to one of many interesting videos on the subject on youtube:
Working from the edition number sequence, I have concluded that the salmon coloured rows show years when a Directory was not produced and does not exist (see the Key at base of the table).
On the Title Page of a Directory it would seem that it was not until 1957 that the Sheffield Directory displayed a copyright and date notice.
As Elaine had previously commented, I personally can only agree with her that it seems that the information in a Directory was canvassed during the year before the year of its title. Obviously that is just my opinion working on the details in each directory regarding the ‘Council Details’ section. However interestingly from the 1951 Directory to the 1965 Directory, the title for this section changes from the ‘year before the Directories title to the year of the Directories title’; to ‘the year of the Directories title to the year after the Directories title’. This seems to occur for the period when the Directory for Sheffield was not consistently produced every year. Because of this, was the data collection/publication pattern altered?