In the end, on the same principle as pick-up lines, the simple, direct approach was best: "Hi, tonight is World Book Night. Would you like a free book?" That was enough to get folks to pause for a look.
Once they paused, I said more. Sometimes I told them how much I liked the book. Sometimes I launched into a short description ("It's a story about a future time when it's illegal to own books. The government doesn't want people to think for themselves."). Sometimes I said it was a science fiction book, "but don't let that put you off." Sometimes I emphasized that it was written in 1951 but the author imagined some things that are weirdly accurate now. I just went with my gut on what might appeal to the person.
Most people readily admitted they didn't read much, so they were squarely in the target audience of "light readers" and "non-readers."
By the end of the night I'd talked with people from India, New York, and Jamaica, from teens to retirees, and families, couples, and friends of both genders. My impression was correct: the Riverwalk was a fantastic crossroads of my town. Sooner or later, everyone shows up there. And it doesn't take long before the beauty of the place puts everyone in a good mood.
During the entire evening, only 3 people refused the book. Anyone who remembers the "seashells" described in the book will appreciate the poetic appropriateness of the fact that all 3 of them were wearing earbuds attached to their iPods.
Life imitates art, in this case.