The Naked Truth: Why Hollywood Doesn't Make X-rated Movies
Kevin S. Sandler
Rutgers University Press
FINALLY IT'S BEEN DONE!!!
Someone has finally written a scholarly work on the subject of the film ratings that is thoughtful and accurate!  This book focuses on the criteria that distinguish R-rated films from those officially rated X or NC-17, but it also gives an excellent overview of American classification and Hays Code activities over the history of cinema.  
Since I have done extensive research on this subject myself, I have been appalled at how much junk has been written in the mass media about it; most writers don't seem to be bothered by glaring factual errors and their own confusion about the subject.  Instead of misattributing such confusion to the supposed inconsistencies of the ratings board (although their criteria do change over time, quite deliberately) this author Sandler correctly sorts the objective from the subjective, assesses the actual film content and key rating decisions, to provide an authoritative work on both the criteria, the process, the politics, and the occasional flubs involved in the ratings process.  
This is the most important, authoritative book on the ratings system since Stephen Farber's 1972 insider work "The Movie Ratings Game" (and that was not written with scholarship in mind).  This book does not go into details about the criteria for other ratings (my own research does that and is still being prepared for publication) but it provides a welcome change from the misguided and biased articles (and films) that have been appearing on the subject for many years.
Finally, a true scholar publishes an excellent work on the subject!!!
Although a previous book "Freedom and Entertainment" was also scholarly and of decent quality (and benefited from the newly available recollections of ratings board chair Richard Heffner, who was in the position longer than anyone else), that book had made some superficial errors and didn't quite convince as being rooted in a thorough and comprehensive understanding of the ratings system.  By supplementing the Heffner material with actual content descriptions and comparisons, this book has filled in the last methodological step that was needed to produce a valid model of the system's operation.  Bravo!!  At long last!!
Finally placed into valid perspective is the long controversy over the supposed favoring of major studios over independent productions, as well as the numerous accusations about inconsistency in the application of ratings (not that the system is devoid of inconsistencies, but they are of a form different from what many have attributed to them).
Unfortunately falling outside this book's subject matter is the recent, widely publicized Harvard study on so-called "ratings creep," which would have made at least a nice footnote in the section in which Jack Valenti defends the rating system by explaining the many conflicting societal forces whose needs it must serve and try to balance.  The blatant problems with the Harvard study included the assumption that unaccountable, pandering film websites would be less malleable in their standards over time than the main body that is answerable to congress and various interest groups.
In the meantime, this book is the most important one currently available on the subject!  Anyone who wants to truly understand the system (and not just criticize) must buy and read this book!  Nothing else comes close, except the very early and out-of-print book by Stephen Farber, called the "Movie Ratings Game" and (in third place) the recent "Freedom and Entertainment" (both of which have many key points summarized and included here anyway).

Awesome job!!!!!!!!!!!!!