This would indeed be helpful.
Perhaps "Original" or "Source Copy" or w/e could be displayed in the Edit Metadata window:
Also, if one were to use the "Add new format for this book" option, there was a checkbox (unchecked by default) labeled "Set As Source Copy"
This file could have the Read Only attribute enabled and could either have some variation in the filename or extension in Calibre's ebook db such that one could have, for example, mobi as the source copy for a book but when then converting
to mobi, the converted file didn't overwrite the source.
Perhaps there could be check boxes in Preferences->Behavior which were labeled things like "Use source copy as default input"; "Source copy can not be overwritten"; and/or "Backup source copy upon adding new book" to give us more flexibility or control over our ebook library.
Perhaps there could be an option under Library Maintenance on the Choose Library toolbar button labeled "Back up source files"
Just throwing ideas out there. I think it would be helpful to make it more obvious which file is the original one and for said file to be more difficult to accidentally overwrite/delete.
For myself, as I have a Kindle, whenever my source file is a .mobi I always rename it to .azw - for one since Calibre can only convert
from azw, but not
to azw (since it's technically the same thing as mobi anyway). I also have MOBI set as first in my Kindle 2 Device Interface plugin.
In this way, I know that AZW (when available) is always my source copy and can not be overwritten by Calibre; since I only have .mobi files sent to my Kindle, I know the copy on my Kindle is from Calibre and thus formatted the same as all my other books; and I can convert from Kindle format to Kindle format without creating problems.
What I do may not be so easy for those who use, for example ePub as both their source copy
and their desired send-to-device format. This idea of setting a certain file as an original/source would solve certain problems.