Powell’s Books in Portland, Ore., piloted an adult summer reading challenge two years ago after hearing from many customers who were jealous of their kids’ contest, which they’ve run since 2013.

Credit: Avon Books. ‘Alice in Wonderland,’ by Lewis Carroll. Credit: Collectors Library. Jacob reading Flannery O’Connor. Credit: Mario Perez. ‘Haroun and the Sea of Stories’ by Salman ...