Interpretation matters in books, for example (7,8)

I think it could be either:

Which length description are you looking for?:

7,8

15*

reading material

I'm a little stuck... Click here to teach me more about this clue!

But I've got no explanation for this one! Could you help me understand?

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app