Many items in one table cell I have a very complicated view, and I can't change the existing field structure (it's an established and functioning site). This is a simplification of the problem, but it has to happen this way, I can't turn it the other way around...

There are two node types (Author and Book), the Book type has an entity reference pointing at the Author.

The view I want to create lists the authors and needs to display all the Book titles in a single column (one author per row, all the book titles in one cell together).

How do I create the entry in the view, so it does that?
You might be able to get the result you want using the Views Aggregator Plus contrib module.

It's still in alpha for Drupal 8, but it allows you to group and aggregate results.