(By "apostate," he means that Haggis rather famously renounced his decades-long cult membership in 2009.)
Cook goes on to say, in part:
The book, which doesn't have a publisher yet, will be called The Heretic of Hollywood: Paul Haggis vs.The Church of Scientology and will explore both founder L. Ron Hubbard's life and Haggis' personal investigation into [current Scientology head David] Miscavige's violence, the enslavement of "Sea Org" volunteers, and forced abortions for church workers, according to a catalog from Wright's agent Andrew Wylie [pdf] that was pointed out to us by a tipster. Full text of the catalog copy is below.
Wright has mentioned the project before in interviews, and indicated that it grew out of a New Yorker story that hasn't yet run.
So there's that to look forward to, too.
Ed. note: the phrase before the "[pdf]" bit in the blockquote is a link in the original, but it looks like an erroneous one as of this moment. Visit Cook's post to see if it has been fixed. The link is now fixed.