The Collaborative International Dictionary of English v.0.48:

coauthor \coauthor\ n.
   a writer who collaborates with others in writing something.

   Syn: joint author.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

co-author \co-author\ v. t.
   1. to to be an author of, together with another author or
      authors; to author jointly with others.
      [WordNet 1.5]

   2. to exist together at the same time.

   Syn: coexist.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

coauthor
    n 1: a writer who collaborates with others in writing something
         [syn: coauthor, joint author]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "coauthor":
   advertising writer, annalist, art critic, author, authoress,
   belletrist, bibliographer, collaborate, collaborationist,
   collaborator, columnist, compiler, compose, composer, copywriter,
   creative writer, critic, dance critic, dash off, diarist,
   drama critic, dramatist, editorialize, encyclopedist, essayist,
   formulate, free lance, free-lance, free-lance writer, ghost,
   ghostwrite, ghostwriter, humorist, indite, inditer, knock off,
   knock out, literary artist, literary craftsman, literary critic,
   literary man, litterateur, logographer, magazine writer,
   man of letters, monographer, music critic, newspaperman,
   novelettist, novelist, novelize, pamphleteer, penwoman, poet,
   prepare, produce, prose writer, reviewer, scenario writer,
   scenarist, scenarize, scribe, scriptwriter, short-story writer,
   storyteller, technical writer, throw on paper, word painter,
   wordsmith, write, writer