[walrus01]

Researching the full extent of the old AT&T long lines network, the AUTOVON network, L3 carrier systems is a very deep rabbit hole to go down. It's a combination of all the major pre fiber Intercity telecom links in north America plus a lot of amazing military grade underground bunkers in some key locations.

[t3rabytes]

Are there any books on it?

[walrus01]

there's a guy who published a photo book:

https://99percentinvisible.org/article/vintage-skynet-atts-a...

but I think most of the info is online and from internet-based hobbyists and retired AT&T long lines people who staffed some of these sites.