Thanks Andreas - I can see that helps a bit, but isn't as elegant as I would have liked.
If I make a copy, then I have another 'Book' object. If I change that to a type "Book chapter", then the author / title details are copied into the respective 'Chapter' fields, not the 'Book' fields, which is incorrect. So for each chapter / section of the book that I create manually I have to cut & paste those fields across.
My use case is that I have (or can usually easily get) bibliographic details for the book which Paperpile can import. I then want to add in Chapter records, sometimes I have them in a format that Paperpile can import and sometimes I have to do it manually. Either way, it is a many-to-one relationship and it feels inefficient to have to manually copy / amend the details of the book many times.
There is also the issue that there is a lot of repeated data about the book in every Chapter record - e.g. title, editors, date, publisher, possibly URL - so should any of those details change it would be messy / impossible to correct them all, leading to inconsistent data.
As there is a clear relationship between a book and its chapters it should be possible to record that relationship in a straightforward way when creating records and for the relationship to be indicated and navigable by Paperpile.
Has this just become a feature request?