Hi.
Good advice on the backups, but I think the comparison with the physical CD is slightly misplaced. First and foremost, as someone commented above, the audiobooks are supplied by Audible. If you buy from Audible themselves, you have an online library, which you can re-download from whenever you like. It is not unreasonable to assume that Apple would be allowed to provide the same service, as they are acting as a conduit for Audible sales.
Secondly, if re-downloading of almost everything else is allowed (including music), then why not the books? The "purchased" section looks very much like it should be offering them and doesn't.
So, no, I don't think Hilda's question is unreasonable. When there is an unlevel playing field, it should be pointed out.
The fact that Apple do not make this clear, with say, "a nice simple message", is indicative of poor support in this area. The way the cloud is described by marketing, gives the impression that all your purchases are backed up. People shouldn't have to go searching through support FAQs and forums to find out differently.
Happily for me, I have always backed up my audiobooks (like everything else on my PCs over the years), but it would be much easier to transfer them to new hardware if they were included the cloud. It's a shame they are not.