I have only looked at fraction of what is available but I have spent a fair amount of time on Pigot and Co.'s national commercial directory for the whole of Scotland and the Isle of Man. This particular edition was published in 1837 and not only does it contain lists of the higher status members of each area and tradesmen but also contains a 'gazatteer' type description of the area.
I came across this particular edition while searching for ancestors of a friend who came from Pitsligo and Rosehearty. There are a variety of file formats that are available that can either be downloaded or read online and all verions can by searched by keyword.
What other commercial directories have you found useful?