It is a Greek plural noun, meaning the books, or the little books.

Sears soon discovered the reason for the plural noun.

We have spelled it with an apostrophe S. Another thing is the possessive of a plural noun.

The ONS style guide for those writing official statistics says: The word data is a plural noun so write "data are".

It is also, and perhaps chiefly, in this case, a pun on the meaning of the plural noun "cenci," "rags," or "old rags."