Thanks for finding that out.
The way I distinguish between the contributors now is that I check which contributors are attached to chapters.
Like I mentioned, in the user permission level both chapters authors and volume editors are ROLE_ID_AUTHOR (a constant used in the code) so we can not use the roles to distinguish the difference. Of course the labels for the roles are different, but since they are editable, those can not be used either because if a press would edit the label, the plugin would not work as expected.
Of course I could just add the contributors to the book level in cases where they are not mentioned as chapter authors. But that would not work in situations where the volume editor is also selected to be the author of a chapter, which is also very likely.
So to be honest, I do not have an easy fix for this.
Edit: just to add a screencapture of the current roles in OMP regarding this issue. Author in the permission level column means that in code level these roles are identical and we can not distinguish between them. Like I mentioned, this is also a problem with Translators.
In my opinion the problem comes from the situation where we try to use the same taxonomy for defining user rights within the system (applies to the users table) and contributors roles (applies to the authors table). These should be separate.