Apple's iCloud - Features section of the website has the following disclaimer:
Previous purchases may be unavailable if they are no longer in the iTunes Store, App Store, or iBookstore.
I think this is an important point to notice. Many years ago, I remember looking everywhere for the Toy Story DVD. I searched online, went to physical stores, asked at Blockbuster. Nothing. I just couldn't find it anywhere.
Later I learned about the Disney Vault. A practice that, as Maryann Wright reports is "a marketing tradition that keeps a selection of films in moratorium for years at a time before strategically re-releasing them with extra scenes and bonus features - at a price." I assume Toy Story was in the vault at the time.
While this is arguably not a good idea these days where people can find anything online, it still happens. It may also happen with books. I don't know.
But given the above disclaimer, it's probably a good idea to keep an extra backup of anything we buy on iTunes, App Store, or iBookstore. It would be pretty annoying to want to re-download a previously purchased movie and not be able to for several years because the publisher decided to remove it.