mmedic;
I've tried to repro but was not able to as well. Here's what reentrant means from Wikipedia.
In the context of this issue, this exception can occur if you have a RelatedEntityList<T> and you are making a change to that list using either Clear(), Add(), Remove() and/or RemoveAt() methods. And while that change is still being made, another change is executed.
In your case, you have a Book with a RelatedEntityList<Isbn>. My suggestion is to check whether there's another Add, Clear, and/or Remove operation that is being executed while you're deleting the Book's object graph.
Hope this helps.