Lost full calibre ebook library following update to High Sierra

Besides deleting a folder of 4000+ movie poster, the update to High Sierra also deleted ALL of my ebooks managed by the app Calibre, all of them tagged and edited and commented on!

When I opened Calibre by dropping a new book on it, the ebook-folder was empty/newly generated.

I have yet to discover what else is missing. But what I do not understand is WHY? Why my posters and my ebooks?!