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what actual use case would this be practical for?
Being brutally honest, none.
It's a "homework assignment" type app, and is far too limited to be of any real use.
For example, you'd have to modify the source code and recompile it every time you bought or gave away a new book!
Tasks like these aren't meant to be used in the real world; they are meant to teach you how to analyse a task specification, design a solution, implement it, test it, and get it working - which requires a particular mindset, a "way of thinking" that many people do not develop naturally, or do not try to apply outside the narrow field in which they do develop it. These are "general skills" which are part of the problem solving process which are applicable to any problem, computing or not.
And skills need to used in order to develop them, just as riding a bike requires you to get on and try to pedal in a straight line without falling over - you can watch as much of the Tour de France as you like, but that won't make your first attempts on a bike for yourself any less painful!
So while it's not actually useful, that's not a bad thing: it teaches you something, even if you don't realize it at the time - and that means that the next, more complex task is easier for you to work on - until you get to apps which are genuinely useful!
So be proud of what you have done - even if nobody will ever use it in anger! :D