To access the shapes, click the + button in the toolbar and then switch tabs from Media to 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:
Change the colour of the shape
Add a shadow
Add a border (with further options to change the colour and size of the border)
Move the shape back or forward
Delete the image
Speech bubbles and thought clouds
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 and circles.