OK, then I've simply to try to install them and see with my eyes
Regarding the thing of Thumbnails I'm asking myself if a compromise could be done, it not possible to show the books as icons? The icon image being the book cover?
I think epub support attributes or if it is not usually the cover is in the ebook itself on the first page...
The ideal thing would be to have a File Manager as separated application, so one could have icons and maybe have the possibility to choose to open a file with a given application for example PDF con Kobo readear, EPUB with CoolReader, RTF with Koreader, CBZ with whatever...