Seems like they could just announce removals 30 days in advance which would give you plenty of time to finish a normal book. They could also stop new downloads of a book during this or a shorter period to prevent people from starting a book without time to finish it. To those that havent started the book, it would just appear that its already been removed from availability. Some kind of a warning of how much longer it will be available on the device would also be helpful as long as its not intrusive.

It's possible for publishers/amazon to build a contract that will protect you in that case while still letting authors drop books from the service.