Reading Lists are made up of sections. If you selected a template when creating a List, you can add new sections, or work with the sections you have. 

Click Add and select New section.

Give your section a title and optional description. You also have the option to enter start and end dates for each section and limit student visibility to this date range by clicking the check box. From drop-down menu, choose where the section should appear in the list. Click Add to finish adding the section.

You can continue to add more sections as needed. Once added, you can re-arrange the order of sections by dragging and dropping.