Yuuko's Past

Yuuko asks Watanuki to get a bottle of sake in the storage room. When he got it, he saw something glowing on the shelf and he saw an old book with a heart-like design on the cover. Watanuki gives the book and asks what book was. Yuuko gives him an opportunity to answer it by himself and gives the book back to him. What will happen to Watanuki after opening the book?