When having bought a book from the iBook store you do not have to pay to have multiple copies – as it supports the ability to view a title from multiple devices, you can download it as much as you want at no extra cost. In effect you have paid for access to your own copy of the text, rather than a physical copy itself, meaning you can download it as much as you want as long as you are the person doing this.
In order to download a book for a second or third time all you need to do is go to the Store and tap the Purchased button to view all items you have purchased. This will then allow you to find the books that you want to download again and do so at no extra cost. This can be used for multiple reasons, say if you want to access the files on separate devices or if you have had to format a device and want to download files again.
It acts as a sort of online backup so that you can access them again should something do wrong, if you lose a device or if something has gone wrong the first time you downloaded the file – say if you lost internet connection. A record is kept of your purchases so that you can access them whenever you want.