Q: book disappeared from ibooks purchase history

Have you checked to see if you've hidden it from the cloud : Hide and unhide purchases in iTunes or iBooks on your Mac or PC - Apple Support ?

If the version of the book that you bought is no longer in your country's store, or if you've moved countries since buying it, then you won't be able to redownload it, you will need to copy it back from your backup of your downloads/library.