Question on sub-libraries

I just got my Nova Pro and transferred several hundred books in epub, mobi, azw, and pdf formats to the device.
Now I would like to sort them all into sub-libraries and am somewhat dismayed that this seemsnto be possible only book by book, which is rather slow.
It would be very useful if one could select or mark multiple books and then move all of them at once into a sub-library.
Anyway, where is sub-library information stored, as it evidently does not simply correspond to subdirectories in the filesystem.

You can select multiple books by clicking this

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Ah, thanks! Will try this right away.