This is one of two books on William Friedkin, the other being the out-of-print Hurricane Billy, which is a straight biography.
Clagett's work is more academic, but it's one of the better examinations of a director and his work, heavily relying on the thoughts and ideas of the filmmakers (collaborators included). The analysis is done through the filter of the thoughts, ideas, intentions, and inventions of the filmmakers. It chronicles Friedkin's influences and work from his television days right on through to Rules of Engagement (this volume is a revised edition, as the original 1990 one only went to The Guardian). The chapters are smartly broken up into two parts: one is a detailed reading of the film, noting the details and imagery, the other is about the making of the film, with comments from Friedkin himself, the best part of the book, and with the many collaborators and players.
The result is a body of work that is consistent with Friedkin's personality and interests, if not consistent in overall quality. This serves as an excellent depiction of a strong personality, an obsessed man, in many ways, and the conflict of directing 'Hollywood' films but with an outlaw element. Friedkin's films do come off as consistent: there is a gritty, cynical attitude, and the endings are not typical--there's hardly a film in Friedkin's catalog that ends happily ever after.
Overall, an excellent book. Friedkin comes across loud and clear in this book: obnoxious, intelligent, honest, funny, and a total character who made some excellent films.