Audiobooks (and tones) are a one-time only download from the store. If you haven't got a copy of it anywhere (it's not on an iOS device, your computer or your backup of you downloads/library ?) and you haven't moved countries since buying it and it's still in your country's store then you can try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption