Merging books and id's
When merging books, the warning in he dialog reads (at least with english language settings) that the ISBN will not be retained. In my experience, this is the case for all id's, for example goodreads id's, amazon asin's or my freshly introduced beam-ebooks id's.
I think either the warning message should be improved, or, even better, the id's be merged as well. At least if it can be done without conflict, and in case of conflicts, the first book should win, as it is with he other metadata.
What is your opinion on this? If we agree on one direction, I could prepare a pull request.