i use a Kobo Clara HD. It runs linux out of the box, the system memory is on a removable SD card inside the case, and the user account is defined in an SQLite database on disk. If you add an empty user account to the database, it removes the “create account” screen and disables any Kobo online services. Then you can install KOReader and upload files over USB as everyone else has said.
yeah see, that’s not the line i would draw for what it *feels* like.