To access the shapes, click the + button in the toolbar then select Media then Shapes.

In Book Creator for iPad, you'll find this under the + button, then ...More > Shapes.

Click on a shape to add it to the page. Once added, it can be resized using the blue handles around the shape, or the green handles for rotation or other manipulation. Or you can click on the Inspector (i) button in the toolbar to:

If you add a speech bubble or cloud, you can add text to it by double-clicking inside the bubble and typing. When you click on the Inspector you'll see the option to edit text settings too. This also works for square/rectangles, circles and rosettes.

Arrows can be a great way to add context to other elements on the page. Use the green handles to move the start and end positions of the arrow. You can change the style, color and direction of arrows by using the Inspector (i) button.