A scrolling sidebar contains content that’s related to the surrounding text but isn’t part of the main text flow. For example, on a page about the moon, you could include a sidebar with information about the first moon landing. The sidebar can contain text, images, and shapes. Readers can scroll through the sidebar, so you aren’t limited by the page size when adding content to it.
Click the Widgets button 
in the toolbar and choose Scrolling Sidebar.
To add text, do one of the following:
Add text: Double-click the widget’s placeholder text, and either type new text or paste text from another location.
Add a text box: Select the widget and click Text Box in the toolbar.
You can resize the text box using its selection handles.
To add shapes and images, do any one of the following:
Add a shape: Select the widget, click the Shapes button 
in the toolbar, and select a shape.
Add an image file: Click the Media button 
in the toolbar, then drag an image file from the Media Browser to the widget (or drag a file from the Finder).
Copy an image from elsewhere in your book or another book file: Copy the image file, select the widget, and paste the image file into the widget.
Important: Images can’t be larger than 25 megapixels (5000 x 5000 pixels) or 50 MB.
Note: You can change the stroke or background color of a scrolling sidebar using the Graphic inspector or the format bar.