In the "Death and Return of Superman" storyline, Doomsday kills Superman and the four other versions of Superman appear. Two of the new Supermen tell Lois that they are the original Superman and support their claims by telling her some of their "memories" as Clark Kent.
However, it is later revealed that a fifth Superman was the original one. He is able to convince Lois of his identity by referencing the story "To Kill a Mockingbird".
Apparently, this was Clark's favorite book and the only other person who knew that was Lois. Does anyone know the comic where Clark originally tells Lois that this is his favorite book?