In Googles case they were digitizing the books (that they did not own), and publishing snippets for search users to help them find books and other material that weren't indexed on the web. The court found they had that right, but did place some pretty strict limits on them.
Still, Google was allowed to keep their database of scanned material despite not owning the originals.
Link: https://en.wikipedia.org/wiki/Authors_Guild,_Inc._v._Google,....
Someone photocopying a book to read on the toilet (and leave the original on their nightstand) isn't engaging in transformative use. They're also harming the market for the work because if they hadn't made this photocopy, they would've had to buy a second copy of the book to get the same benefit.
The situation you describe is more akin to "format shifting" or maybe "space shifting", which is converting copyrighted material to other formats or places as a backup, for preservation purposes, or just for convenience. It is legally protected in the US, and most of the rest of the world. I believe that was settled due to early litigation with VCR's, but there are mountains of relevant cases at this point.
I do recall reading that the UK recently changed their laws in that regard, and like most copyright law it can depend on the specifics of the situation and even (*yuck*) intent. So it's always worth checking if you're unsure.
