This occurs when a writer either knowingly or unknowingly refers to an idea or section in another book without explicitly quoting it.

This is known as an allusion. An allusion is a figure of speech that refers to a well-known story, event, person, or object in order to make a comparison in the readers' minds. For instance, imagine you were reading a book about a stubborn character. The author might refer to this character as "as stubborn as a mule," without mentioning that it's a common saying. This is an allusion to the common saying, and it helps the reader understand the character's stubbornness. It's a way of letting the reader make the connection without spelling it out for them.