Organising Apple Books Collections

As an author I do a lot of research, as well as recreational reading. It would be very useful if I could nest my Collections to make things easier to find (e.g. the main collection for 'nature' might contain 'flora' and 'fauna' as sub-collections). I know that Apple do not monitor this community, and I have submitted a feature request, but I wonder if anyone knows of an add-on or third party app to provide this feature?