Guesswork probably isn't the best option. You may know by now, but for future readers...
If you have the means, the best option would be to look for local businesses in the area in Scotland where said book deliverer lived. Was there a publisher's printing office in town? Or only bookstores? In other words, find out what kind of businesses were in the area that centered around books. There were probably directories of the town and if all else fails, thumbing through newspapers during that time would reveal clues as well.
If there is a printing factory for a book publisher there, then he could have delivered boxed books in bulk to local bookstores.
If there isn't one, then he could have worked for a local store that delivered books to wealthier clients directly.
I think it's probably one or the other.