The Activity Book consists of the instructor pages (first half) and the Student Pages (perforated second half). You need one Activity Book so that you have the instructions. If you have more than one student, you can purchase a loose leaf set of Student Pages for the second student. It saves you the expense of purchasing two activity books or making copies.
You can read more about The Story of the World product family here: https://peacehillpress.zendesk.com/knowledge/articles/360024799074/en-us?brand_id=408728