But that is simply establishing his terms of reference. The meat of the book, and its entertainment value, lies in its extended examination of six leading companies - Paramount, M-G-M, Warner Brothers, Fox, RKO and Universal - along with a chapter on ''Small Studios'' (which may sound as though it promises a tour of Poverty Row, but is in fact largely devoted to Columbia and to two celebrated examples of what Mr. Mordden calls ''the one-man studio,'' Samuel Goldwyn and David O. Selznick).

Take Paramount. It was a studio that continued to bear the impress of its founder, Adolph Zukor, long after other men had begun bossing production on site: a studio that respected directors, and tolerated individualists (Ernst Lubitsch and Preston Sturges, among others). Its specialties were society comedy and high style, and on occasion broad style, too - it was the home at various times of Marlene Dietrich, the Marx Brothers, Claudette Colbert, Mae West; but it frequently failed to capitalize on the talents of its stars. After its early days it lost the art of making good musicals, but it did know how to put across Bing Crosby.

Compare and contrast Metro-Goldwyn-Mayer. M-G-M favored producers; it was the stamping ground of the great authoritarian, Louis B. Mayer; for directors, it relied heavily on what Mr. Mordden calls ''the inspired hack.'' No other studio did so much to mythologize its stars, or had as firm a grasp of how much depended on teaming the right stars and then reshuffling the teams, on ''vitality of combination.'' It was the studio of ''The Thin Man,'' ''Mutiny on the Bounty'' and ''The Wizard of Oz,'' but perhaps above all the studio of the all-star ''Grand Hotel.''

These are only the barest outlines of an analysis rich in example and anecdote, with excursions on everything from the personality of Irving Thalberg to the cinematic career of Tarzan. Mr. Mordden even finds time to amuse himself by imagining what a quintessential early Paramount film, Lubitsch's ''Trouble in Paradise,'' would have been like if M-G-M had made it. (Robert Montgomery and Norma Shearer rather than Herbert Marshall and Kay Francis; swank but no bite.) There is an equally thorough sifting of the main themes in Warner Brothers movies, with special emphasis on their sense of mean streets and bitter social realities (most obviously in gangster films or in a movie like ''I Am a Fugitive From a Chain Gang,'' but even, at times, in Busby Berkeley musicals). Elsewhere Mr. Mordden reflects on the folk values of Fox, the Art Deco sophistication of RKO, and the predominant conservatism of Universal - a conservatism that didn't prevent that studio from springing some major surprises (''Frankenstein,'' ''My Man Godfrey,'' the 1936 ''Show Boat'').

Mr. Mordden is the author of 17 previous books, most of them dealing with either film, theater or opera. He has organized his material with a practiced hand; his comments are perceptive, his judgments strike one as trustworthy, and he enlivens his text with intelligent asides - on the debt that the plot of ''Gone With the Wind'' owes to Thackeray, for example - and some agreeable trivia. How many of us would have guessed that Adolphe Menjou was born in Pittsburgh?