Hi Russ,
I have to asked that question vice versa, have books ever been unstructured? :-)
No I'm not sure. That's part of a script I wrote for a customer which uses FM12/FM2015 in an unstructured environment, and that was the only way I was able to solve that problem.
And I think customer also works with unstructured FM variant and he never complaint.
But I think Folders and Groups are features which came with FM9. And perhaps starting from then books have this "XML"-logic.
Markus