The book directory is “e_book”. Just putting that on your card should be enough.
The complete list of directories (excluding any files) looks like this.
. |-- e_audio | `-- e_record |-- e_book |-- e_image | `-- e_screen |-- e_note |-- e_notepad |-- hwsys | |-- hwdb | |-- hwidx | `-- temp `-- temp
If you go into the settings menu, on page 3 there’s the option to reset to factory default and / or clear all temporary files. I don’t know if that will help recreate the filesystem. There’s also a reset button on the back, but I guess that just resets the hardware – not the SD card.
Hope the above is useful.