In which case should book authors be written?

Question

At the beginning of every book, there is the name of the author followed by the title itself. However, the case, in which the author's name is written may vary. Let me illustrate with two examples:

1. Ioannis Kepleri Harmonices Mundi, which is the genitive, and
2. Carolo F. Gausso Disquisitiones arithmeticae, which is (probably) the ablative.

My theory is that the first example means The Harmony of the World of Johannes Kepler, but the second is Arithmetical investigations by C. F. Gauss. Is this correct?

And is there some "righter" option, or can you use both? My guess would be that both are possible, but the ablative case seems to be much more frequent.

Answer 1

It depends on context.

You could use Medea Ovidii (Ovid's Medea) in most contexts. In the title page of a book, it is typical to write something like Medea actore ovidio (Medea, the author being Ovid). This is an absolute ablative as mentioned in fdb's answer. It would also be grammatical to write Medea ab Ovidio scripta (Medea written by Ovid). A plain ablative is not grammatical. If you are listing authors and their books, you can simply use nominatives: Ovidius: Medea. This only works if the names are not part of a sentence, but items on a list.

In my experience the most common choice for title pages is auctore with ablative. If you want a safe choice that works in just about any context, use the genitive.

Answer 2

The title page of Gauss's book says "auctore D. Carolo Friderico Gauss". It is an ablative absolute: "the author being C.F.G." Without "auctore" it would make no sense.

https://en.wikipedia.org/wiki/Disquisitiones_Arithmeticae#/media/File:Disqvisitiones-800.jpg