If you no longer have the file, is it possible to reorder an Ibook using the original order number
Can I reorder an Ibook using only the order number, since I no longer have the file?
If the book is still in your country's store, and you haven't moved countries since buying it, then you should be able to redownload it for free via the Purchased tab in the iBooks app on an iOS device, and/or via the Purchased link under Quick Links on the right-hand side of the iTunes store homepage on a computer's iTunes (iBooks app on a Mac if it has Yosemite or Mavericks on it).