I'll leave a couple of excerpts, since I'm not sure if the book is still in print:
[The Exorcist], or its ambience, reminded me of The Godfather, both being afflicted with the same pious ambiguity. Ambiguity is not quite the word, for the film's intention is not at all ambiguous; yet, hypocrisy is not quite the word, either, since it suggests a more deliberate and sophisticated level of cunning. The Exorcist is desperately compulsive, and compulsive, precisely, in the terror of its unbelief. The vast quantities of tomato paste expended in The Godfather are meant to suggest vast reservoirs of courage, devotion and nobility, qualities with which the film is not in the least concerned - and which, apart from Brando's performance, are never present in it. (And, at that, it is probably more accurate to speak of Brando's presence, a pride, an agony, an irreducible dignity.) The Exorcist has absolutely nothing going for it, except Satan, who is certainly the star: I can say only that Satan was never like that when he crossed my path (for one thing, the evil one never so rudely underestimated me). His concerns were more various, and his methods more subtle. The Exorcist is not in the least concerned with damnation, an abysm far beyond the confines of its imagination, but with property, with safety, tax shelters, stocks and bonds, rising and falling markets, the continued invulnerability of a certain class of people, and the continued sanctification of a certain history. If The Exorcist itself believed this history, it could scarcely be reduced to so abject a dependence on special effects....
The mindless and hysterical banality of the evil presented in The Exorcist is the most terrifying thing about the film. The Americans should certainly know more about evil than that; if they pretend otherwise, they are lying, and any black man, and not only blacks - many, many others, including white children - can call them on this lie; he who has been treated as the devil recognizes the devil when they meet. At the end of The Exorcist, the demon-racked little girl murderess kisses the Holy Father, and she remembers nothing: she is departing with her mother, who will, presumably, soon make another film. The grapes of wrath are stored in the cotton fields and migrant shacks and ghettoes of this nation, and in the schools and prisons, and in the eyes and hearts and perceptions of the wretched everywhere, and in the ruined earth of Vietnam, and in the orphans and the widows, and in the old men, seeing visions, and in the young men, dreaming dreams: these have already kissed the bloody cross and will not bow down before it again: and have forgotten nothing.