As a tangential niche-tourism shoutout: https://www.telcomhistory.org/ConnectionsSeattle.html

Granted, most of it is a bit newer than the heyday of telegrams, but there's a little bit of overlap.

You may have already read it, but portions of this sprawling (and very very good) essay by Neal Stephenson get into the topic: Mother Earth Mother Board - https://www.wired.com/1996/12/ffglass/