Anyone who is serious about genealogy, certainly here in the UK, will be familiar with Kelly's Trade Directories!
These wonderful tomes of knowledge carry trade information regarding business and their proprietors and were started by Frederick Festus Kelly. In 1836, he was chief inspector of letter carriers for the General Post Office and took over the publication of the Post Office London Directory. Eventually he acquired the copyright himself and in the following years he expanded the business, branching out to other regions. In 1897 the company was renamed Kelly's Directories Ltd, and from then until the 1970's annual editions appeared for most country directories.
Even just to look through the earlier editions is intriguing, with all the contemporary advertising. Local Libraries usually have a set for public view - they are well worth a look! (and if any of your family were 'in trade' you might even find out the odd snippet or two of information about them!)