Card libraries are used for defining decks and other general collections of cards in the kit (e.g., unlockable cards).
Card libraries in the kit are Scriptable Objects. You can create a new enemy asset by right-clicking on your Project view and selecting the Single-Player CCG Kit/Templates/Card library option. Once you have created a new card library asset, you can edit it using the kit's editor located in the Tools/Single-Player CCG Kit/Editor menu option and selecting the Card libraries tab. This will bring you to the following view:
Here you can set a name for the library and define the list of cards it contains (including the number of copies of every card).