"The Exorcist" and "Legion", The Screenplays (1998)
EXORCIST [THE] & LEGION - Screenplay by William Peter Blatty
1998 - Faber & Faber - ISBN 0-571-20015-X
This has the un-cut shooting script of "Legion" before the requested re-shoot. No Father Morning character and no exoticism. It's very good in it's original form. A very good read, and I recommend it to any Blatty fans out there.
From the book's back cover:
Here for the first time in print are Blatt's electrifying screenplays: The Exorcist, directed by William Friedkin in 1973, and for witch Blatty won an Academy Award; and Legion, subsequently filmed as The Exorcist III. Included are the full texts of The Exorcists legendary "Lost Scenes" and excised dialogue, plus the controversial climax Blatty intended for his extraordinary sequel.