The Aluratek Libre Pro and the Ectaco jetBook are the same hardware, including the
The file system that is being used is FAT, and you should use FAT32 on your SD card.
(I also format the internal memory to FAT32 from the FAT16 that it has from the factory.)
You can find the limitations for the various FAT file structures in the online descriptions.
P.S. For these particular devices you also want to have no more than three levels to
your file structure. So if you had a "Mysteries" folder in the root that had folders for
each of your 2001 authors, and then had series sub-folders under the author folders
you could be having problems because of that.