Apple Audiobooks

As far as I know they are all supplied to Apple by audible.com (I haven't seen any that aren't). That book does appear to have quite a few versions here in the UK, but of them looks like it's narrated by a different person.

Audiobooks bought from iTunes are currently a one-time only download from the store (if instead you'd bought it direct from audible.com then I think that you can re-download it from them) - you will therefore need to sync/copy it to each device/compute that you want it on.

Yes, you can copy them back to your computer's iTunes (File > Devices > Transfer Purchases should copy them over from the device to the Books part of your Library), and you can then sync them. And yes, if you lose all of your copies of it then you will need to pay again to re-download it.

For music Apple have presumably done deals with the record companies to allow re-downloading (and that isn't yet possible in all countries) - and you will only be able to re-download them as long as they remain in the store, so for music (and all other downloads) you should keep your own backup copy, just in case that they are removed from the store. I assume that for audiobooks that it's audible.com that are requiring the one-time only rule.