I was thinking along the same lines as ThatFanficGuy that classics are the best single book. The other side of that coin is nobody on the fence about reading more is going to pick up a book thicker than it is wide.
I'm going to go with Into Thin Air by Jon Krakauer, even despite the valid criticisms against it. It's an easy read and a compelling story, and the author is somewhat known in pop culture (because of Into The Wild which I can't pick no matter how easy to read or compelling it is). It's the kind of story that might make someone think "maybe if I should try this difficult thing, and it's ok to not succeed as long as I don't die." That little push can help people.