Apps tend to be quite small (less than a megabyte), and if your pictures (and presumably music and video files) as well as ebooks are on your SD card, it means your Library should be empty.

But I suspect it's not.

Note that if you "Add to Library", the eDGe is copying the files into a protected area of Internal Storage, i.e., you'll have the same files in two places - Internal Storage and your SD card. If you have items show up under "Library", then that's why you're running low. You can read my recent post here for some options and advice.