With the release of Book Creator 3.4 we added shapes to the toolset.
Accessing the Shapes menu
Tap the + icon in the toolbar, choose the More tab and you'll find the Shapes menu.
Tap on any shape to add it to the page.
Editing a shape
Once you've added a shape, you can edit it in a few ways.
Tap on a shape and you can drag the blue control handles to resize and reshape as you want to.
You'll notice some shapes also have a green control handle.
For arrows - this will change the size of the arrow (try rotating the arrow in any direction too).
For rounded rectangles the green control handle changes the amount of border-radius (making the rounded corners more or less subtle).
For the speech bubble use the green control handle to adjust the position of the arrow coming from the bubble.
Changing colors, shadows and border
Tap on a shape and then tap the Inspector (i) in the toolbar and you can change the fill color, and toggle the border and shadow on and off. You can also adjust the color and width of borders.
From the blog: