How to delete a book in a library when its bought?

We can't help you, partly because nobody deletes books from a collection when it's bought in the real world - they set the stoick level down one each time a book is sold. When the stock level reaches zero, there are none in stock.
That way, when a delivery comes in, you just add the delivered quantity to the stock level and you are good to go.

But mostly because we have no idea what you code looks like, how it works, what you are storing, and how you are storing it - and even in an ancient language like C there is no "one right way" to store a collection of books!

Start by looking at how you store your data at the moment, and how you add an item to that collection. Deletion is likely to be the reverse of that process!