What is the tag question of book were lost?

The tag question for "Books were lost" depends on whether you want to express certainty or uncertainty.

For certainty:

* Books were lost, weren't they? (This implies you are fairly sure the books were lost)

For uncertainty:

* Books were lost, were they? (This implies you are not sure if the books were lost)

You can also use a more formal version:

* Books were lost, were they not?