[falcolas]

AKA, the original whispersync.

Yup, this was once a thing - you didn't need wifi for sync or downloading books at all.

[iso1631]

Kindles at one point apparently came with free cellular access

https://xkcd.com/548/

[int_19h]

They still do - it's an option on more expensive devices (Paperwhite and Oasis).

[sct202]

I had a kindle keyboard and it had 3g. It worked in a bunch of countries--slowly though. I remember reading blogs where people were taking the sim cards out and tethering using them.