It is to be used without any cover. But you answer my question anyway, thanks!
I read here about Etsy, which I don't know before, I may buy something like that:
https://www.etsy.com/listing/1624063...-kobo-glo-aura
[Edit] I look at some other sleeves and changed my mind, there are some beautiful in leather, and it will protect the ereader better. ^^