Why must I repay for a book previously purchased
WWhy must I repay for books previously purchased .but have disappeared from my cloud
iPad Air, iOS 8.1.2
What sort of book, ibook or audiobook ? If it's an ibook, and it's the same version that you bought and you haven't moved countries since buying it (and you are logged in with the same account that you bought it with), then you should be able to redownload it for free via the Purchased tab in the iBooks app on your iPad - if it doesn't show there then check to see if you've hidden it : iTunes Store: Hiding and unhiding purchases - Apple Support
If you've moved countries since buying it, or if the publisher has removed the version that you bought and replaced it with a new book, then you won't be able to redownload it. In which case do you have a copy of it in the Books part of your computer's iTunes library (or iBooks app if using a Mac with Mavericks or Yosemite on it), or your backup of your downloads/library ?
If it's an audiobook then they are one-time only downloads from the store.