I also can't find a guide anywhere by searching, but here are the steps to set your library folder:
- When you open Librera, and are looking at the main screen, tap the three lines/hamburger menu button on the top left.
- When the confusing settings pane slides open, look for the aqua link near the top that says "New folder".
- Tap "Add"
- Tap the folder your books are in.
- Tap the "Select" button.
- Tap "Find"
After you've done that, then the folder should be auto scanned for books going forward.