The idea that you could use cards to catalogue books goes back at least to the eighteenth century when people began to use for this purpose the backs of playing cards, which were often blank. Edward Gibbons also adopted this method. For a description of Gibbon's approach, see this old New York Times article, for instance. Here's a picture of a display of some of the cards:
This is in reference to the previous entry. It is also in reference to other accounts of the history of index cards, such as that of Markus Krajewski's ZettelWirtschaft. Die Geburt der Kartei aus dem Geiste der Bibliothek (Kadmos Verlag, Berlin, 2002), for instance, which gives a partial description of the role of playing cards and other matters in the history of index cards.