What is an example of allusion in the novel, All the Bright Places?

The first time that Violet and Finch communicate with one another using Facebook, Finch initiates the conversation with a quote by Virginia Woolf from her book “The Waves.” Finch later looks for this book in both the Bookmobiles and at the bookstore at which his mother works but is unable to find it. During one of their last dates together after Finch has taken to his closet, Violet gives him a first edition copy of this book for his birthday.