I am not certain if this would help. It is an app that allows you to change the formate of e-books. So if you have an e-book in X format you can change it to the kindle format and vise versa. I have used it to change to kindle from an old Sony e-reader.
So if your problem is you have different formats this will work. kindles only read kindle format and there are many different formats they used to be device specific, so if what your are transferring is not from amazon (kindle format), your kindle will not be able see/read it.
When I did it, I used it on my laptop to reformat my books, then I synced the kindle while attached to the laptop to transfer them over. Now I keep it and also use to keep a copy of all my books just in case anything should ever happen to my Amazon account.
So in your case. Download Calibre (proper one for your system), then plug you tablet into you computer open Open Calibre and use it to navigate to your e-books, select one, and then select what you want to do with it. I think (if I understand what your doing) would select change from X format to kindle, then click it and wait a moment. Next up create a folder for keeping your converted file (calibre will let you do this in there interface). Just keep repeating till you have all you want (can't remember if you can do it in bulk). Next pug in the kindle, open calibre and select to transfer over.
This was the best app I could find when I did this a few years ago. I still keep it as I find it has many uses when it comes to working with an e-reader. One nice thing you can do with it is remove the encoding so if you and (in my case) your sister both have one you can loan your book to them, download remove the DRM, then email and they can upload via the app to there kindle.
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