So for those who don't know, "waste paper" had a very different meaning in the past.

Back before wood pulp, paper was super expensive. Like so expensive you'd fold your letter up and seal that rather than use an envelope. If you had to jot down a note, like a shopping list before going into town you'd cut it off of a larger piece of paper to save money. Seriously, a lot of historic documents are weirdly small because they were being cut down from the standard sizes.

The exception to this was "waste paper". It was small scraps that were offcuts from the paper mill. These would frequently be bound into small, affordable books that could be carried in a pocket. These were ideal for small, mundane writing like notes and lists.

This shop is advertising that they carry these small books.