Not only did William Peter Blatty write the second sequel to this film, he directed it as well
[DOC] [TLE] An interview with William Peter Blatty ( Taken from Fear Magazine ) [PAR] Before the Publication of his novel, The Exorcist, in 1971, William Peter Blatty had 
... film in 1973, directed by William Friedkin, produced and written by Blatty.