The NYT had an article about the German telephone directory listing of actress/singer Marlene Dietrich. The article included a scan and explanation of the directory page.
Article:
The article mentioned that some old phone books are being digitized for ancestral research. This seems very interesting.
I knew someone with a 50 year old phone book of my city and going through the old listings was quite interesting. For families and people that I knew, most were living in the old neighborhood.
Also of interest was a comparison between the old directory and the current one. We could see for some names a man in the old neighborhood vs. a woman's name in a senior apartment, presumably the widow of the man in the old listing. We saw a surprising number of listings that hadn't changed in 50 years, and many where the name/ address was the same though the number was different. In the new directory we saw new ethnicities that weren't in the city 50 years ago.
In a very old phone book, I saw a church whose number was unchanged from 1923 to today. Back then it was merely '29', today it is nnx-0029.