If you see the house icon, "KOReader Home" and "+" on top in the app you can click on the "+" and then choose the folder with your ebooks and set this as "HOME folder".
There's also the user guide:
https://koreader.rocks/user_guide/
Plus there's the wiki:
https://github.com/koreader/koreader/wiki
What I dislike about KOReader is that it's rather slow showing all the book covers plus I can't scroll in the home folder view but have to swipe.
I like Libera (librera.mobi) more, it's much faster in the library view and I can just scroll through the cover list. Plus I can use dictionaries in the same file format as with KOReader via the app ColorDict.