Thank you. But that approach is the usual one for specific files or, eg, documents. While I had lost some files, my initial concern was the loss of the organisation structure provided by "Collections".
I just found this post re missing Collections from 4 years ago:
"Here they are stored in :
~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books"
Obviously there's no iBooks any more but I was able to at least restore my missing files by following this trail. However, the organisation provided by "Collections" is still missing and so I'll just have to reorganise things from scratch again.
That's not the end of the world, obviously, but the bigger question is (nb Apple if you're reading this) why did the "Collections" structure and about 50% of the files disappear in the first place ... I certainly did absolutely nothing to cause such to happen. Frankly, I do not use "Books" very often as I think it's dreadful software, and that's being polite. Furthermore, here in Hong Kong there's no Books Store. But if something like this ever happened to eg my Music library, I'd be really "DISAPPOINTED"!
If anyone one out there has any idea of how I can recover my "Collections" structure, please advise. If I don't hear anything within the next 48 Hours, I'll just start the laborious process of reorganising everything.