What would it be?

A well known classic or a more obscure title?
Everyone has a favorite that they go back to, that made a huge impression, or even inspired them into action.
What would be yours?

For years I would have said "To kill a Mocking Bird", but now would stuggle to choose between it and "A Thousand Splendid Suns". "Wide Sargasso Sea" by Jean Rhys would be up there too.