“Books aren’t about ‘real life,’” a minor character says early on in Jeffrey Eugenides’s new novel. “Books are about other books.” In a clever literalization, The Marriage Plot is very much about other books. Eugenides announces this on the first page, in an inventory of Madeleine’s bookshelf, and as the novel progresses and the books mentioned or quoted or discussed pile up, the book’s bookishness is only confirmed.