The Collaborative International Dictionary of English v.0.48:

ghostwrite \ghost"write`\, ghost-write \ghost"-write`\v. t.
   To write (a book, article, speech, etc.) for someone else; --
   the written material appears under the name of the person for
   whom it was written.

   Syn: ghost.
        [WordNet 1.5] ghostwriter

WordNet (r) 3.0 (2006):

ghostwrite
    v 1: write for someone else; "How many books have you
         ghostwritten so far?" [syn: ghost, ghostwrite]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "ghostwrite":
   act for, author, change places with, coauthor, collaborate,
   compose, crowd out, cut out, dash off, displace, double for,
   editorialize, fill in for, formulate, free-lance, ghost, indite,
   knock off, knock out, novelize, pamphleteer, pinch-hit, prepare,
   produce, relieve, replace, represent, scenarize, spell, spell off,
   spook, stand in for, subrogate, substitute for, succeed, supersede,
   supplant, swap places with, throw on paper, understudy for,
   write