The Collaborative International Dictionary of English v.0.48:

ghostwriter \ghost"writ`er\, ghost-writer \ghost"-writ`er\n.
   One who ghost-writes (a book, article, etc.) for someone
   else.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

ghostwriter
    n 1: a writer who gives the credit of authorship to someone else
         [syn: ghostwriter, ghost]