Not a Lawyer, but this seems like work-for-hire.

https://en.wikipedia.org/wiki/Work_for_hire

I assume Germany has something like this (possibly a EU requirement). It would cover more than just ghostwritten books.

Most software is written in a similar manner. Microsoft didn't write windows, lots of ghostwriting programmers did.

The author never had the rights, as they worked under contract, so they don't need to sign them away.

But we also the right to be named as author is not a requirement to be named as author.

This is handled in the employment contract. The "Urheberrecht" is not transferable only inheritable, but you can grant "Nutzungsrechte" which means "rights of use". So in your contract you just grant your employer unrestricted and exclusive rights of use.