It's really simple to copy and paste elements in Book Creator.
You can also lock items in place too - which is a useful thing to do if you're a teacher creating template books for students.
You just need to right-click on an item to bring up the contextual menu. From here you can cut, copy, paste or lock the item. You can also layer it (move it in front of or behind other objects).
When you're ready to paste, simply right-click again and you'll see the option to paste comes up - click it and paste the item wherever you want it to go. In Book Creator, you can copy and paste any item, including audio and video.
BONUS: If you're working with a keyboard you can use keyboard shortcuts to copy and paste: CTRL/CMD X to cut, CTRL/CMD C to copy, and CTRL/CMD V to paste.
Book Creator for iPad
In Book Creator for iPad, you perform a long tap on an item in order to access the contextual menu.