For books that are checked out from the library, I like the idea of having the option to enter a "due date".

That way, you could easily see on your home page/dashboard when you need to return a certain book without needing to cross reference emails or receipts or another app like Libby.

It could also be fun to tie this into the reading goals calculation. For example, if I need to return a 400 page book in 20 days, it could recommend how many pages I'd need to read per day to finish before the due date. (Also optional most likely? Since I know some folks prefer not to see the progress bar)