I thought I must have missed all of your discussion on the Potato book, so I am delighted that apparently it can be still ongoing ......
On a quick perusal of the book, I could find no communication from Sophie. The author(s?) was so clever that I felt like Sophie had written to Juliet many times. For instance, when Sidney broke his leg in Australia, Juliet writes to Sophie "I only got a cable, so you know more than I do. But whatever the circumstances, it's absolutely ridiculous for you to consider flying off to Australia." and again when she wrote to Sophie from Guernsey "Yes, I'm here." In most of her letters to Sophie, I felt like she was answering a letter from Sophie, but it isn't there.
Isola's quote was "reading good books ruins you for enjoying bad ones." I do not totally agree with that, although if I start into a poorly written book, I often do not ever finish it (my reading time/energy is too short).
Clarice