The Martian is a great gateway for casual readers into sci-fi. One person I gave The Martian to loved it so much the next week they asked me for another recommendation. I recommended John Scalzi's Old Man's War.

He's now reading the Foundation series.

I loved it so much that I still haven't seen the film. I don't want the book spoilt. :(

(and I'm a life long sci-fi reader!)