@Sidetrack - indeed, tag type fields would be treated specially to append data to them, not overwrite.
I think the bigger complication now around a ui for this is if trying to support only applying updates to some books.
Either I stick to it applying to update all matched books (which I see as the majority use case). And if you don't want one books data to be overwritten, then you just clear any match for it/ leave it unmatched.
Or I make it a selection based action (like all the others are) with a right click menu option and button on the right side. So this "Update metadata" option will give you the dialog to choose which columns to update, remembering your choice from last time. I can see that working quite well, though you will have the complications of if the user changes their mind etc. or the user could just clear the match and start again.
There are pros and cons with both. The second option is more flexible, but more prone to user error. That flexibility means if the user forgets to select a row it won't get updated.
Any thoughts welcomed.