J.J. Abrams is the “creator” of S.. J.J. Abrams came up with the concept for S. when he discovered a book on a bench outside LAX in which someone had left a note for the person who'd find it. All of the marginalia and loose material in S. was inspired by Abrams' reflections on the ways in people connect through books, a singular book. J.J. Abrams approached Dorst by asking him what kind of book he'd want to create using a story plot within the margins of a book as a structural conceit. They then spent the better part of a year discussing the characters, motivations, story arcs, and other elements, while Dorst did the bulk of the writing1). Abrams' name goes first in the bibliographic entry for the book.
Abrams also produced another work, Bad Twin, which was supposedly written by a character on the show “Lost.”
Here is a Huffington Post article on S. and J.J. Abrams. Mentioned in this article is the word, “ephemera.” The etymological and historical background on this word and what Abrams says about it is weaved throughout the pages of S. This Wikipedia article on the word is helpful.