What is an example of allusion in the novel, The Unhoneymooners?

In the book, Olive makes more than a few references to Harry Potter to Ethan without thinking that he would understand her references. When he does, and makes comments back to her, she is shocked and surprised that they have anything in common. Their shared interest in books is one of the qualities that attracts Olive to Ethan even more as she realizes they are more compatible than she had initially believed.