I used to ‘check out’ books from the library on audio CDs.  These were nice but invariably I would get one with a defective CD.  The CDs soon gave way to books on a self contained digital playback device.  These, in turn, gave way to MP3s down loaded directly to my smart phone.

And while these MP3s are handy to use, you will never have an overdue book because they automatically expire after 21 days.  So, if no one else is waiting to check out the book, you can renew it for another 21 days.  But if some one else wants your book, you are just out of luck.