[kovidgoyal]

nairbv as long as you choose one file format all the metadata will be stored consistently, because you will ensure that while converting to that format. Various formats zipped up is not a single file format, its various file formats zipped up.

So to re-iterate, you will need to write converters from various file formats to your single file format and these converters are going to have to be able to read metadata from all these various file formats as well as converting the content itself. That is the hard part. Writing a converter from some format (even epub) to any other format is trivial by comparison.

Now I've already substantially solved this problem in libprs (the only major ebook format I dont read metadata/convert is .mobi and I've chosen to store the metadata not in a file format but in a database and when you export files from the database, the metadata is written to an OPF file.