Books that have changed you, stuck with you, and/or that have made you think differently – classics edition

I’m looking for a book that fits the description above for a friend as a gift. He’s currently reading Crime and Punishment, and has also read The Bell Jar. But other than that, I’m not sure. He’s very into philosophy and philosophical conversations, very “what makes ‘right’ right and ‘wrong’ wrong?”

Any help is appreciated!