I want to use The Time Traveler's Wife again for this, but seeing as it's already been used, my choice today is Atonement by Ian McEwan. That's if it's not considered a classic already, I remember studying extracts from it in A-Level English lessons.
What books would you call future classics? What is it that even makes a 'classic' novel? Join in the #bookaday conversation over on Twitter!
You can have a look at my other choices here.