To Kill A Mockingbird.
The Hobbit.
Pilgrim’s Progress.
Just to know what great literature can be, to get a feeling for it. Also, generally, you should be a better person for reading each book above.
I enjoyed:
As a child:
Jayne Erye
Withering Heights
Little Woman
As Adult:
Mere Christianity
The Coming Economic Armageddon
Case for Christ
Fiction:
Screwtape Letters
Ezekiel Option
Left Behind Series