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]