Uploading Kindle files from iPad to WMD

I am trying to upload Kindle books from my iPad to the Wireless Media Drive, but I only seem to be able to find photos, movies, and music in the SanDisk app. If I use the Files folder in the app, it tells me that I have no content. How can I upload non-photo, non-movie, and non-music content to the drive? Thanks.

kindle books are not supported. this is an iOS limitation. iOS sandboxes anything other than the camera roll. 

Thank you!