>Furthermore, Amazon hasn’t cracked the code on hyper-efficient GB into KB lossless compression only to squirrel it away only for use in voice assistants.
They could do speech recognition on the device and then ship off the plain text. I don't think they do this, but it is most certainly within their technical ability.
As a practical example, I have a copy of a 458,045 word audiobook on my computer and I just downloaded a copy of the e-book. The audiobook is just over 1 GiB, while the plain text of the e-book compressed with bz2 comes in at 800 KiB.