[daphne]

There is a folder for Kobo in Accounts 1000 > appdata named com.kobobooks.air + a bunch of other characters, but it file browser says it's empty. I suspect the files are hidden. There may be another file browser that would be able to see hidden files. I'm using the .99 File Browser app.
Have you tried looking with the PB connected to your computer?
The books must be stored on the PB somewhere because the books can be read when not connected to the internet.