Archives of Ancient Sumer (c. 4500 – c. 1900 BC) catalogued their holdings of clay tablet texts via the incipits (the opening few words), and the incipits are listed on catalogue tablets for easy reference.

An example of the list:

Honored and noble warrior

Where are the sheep

Where are the wild oxen

And with you I did not

In our city

In former days

Of course, this is an incredibly inefficient system, as the incipit generally tells you nothing about the text that follows--and it depends on the librarian already having a working knowledge of everything contained in the library and the catalogue of incipits is only really good for a memory aid.