If there are unsaved changes in an active book creator project, they cannot be merged with a saved project using "Open in book creator". The options are "Overwrite", and "Append".
IMO, there should be a third option, "Merge", or the "Append" action should check for duplicated lines by default.
The chapter organization makes this slightly non-trivial. I won't tackle this problem because bug 44185 might solve the issue more elegantly.
Version: master
Severity: enhancement
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=44185