Add the option for key-value tag pairs and let users create their own keys for their whole library and fill in their own values as they like. For example: a user can make "recommended-by" a key and optionally fill in values for each book by whoever recommended the book. You could then search and sort values within the "recommended-by" key, so you could see which books were recommended by whom. Other examples: {"location": ["India", "France", "Brazil"], "identity": ["trans", "sapphic"], "would-read-again": ["yes", "no", "maybe"]}