Indulged in moderately and with a sense of proportion, the reading of 50-year-old newspapers can be a harmless, perhaps even mildly beneficial pastime: Americans were a funny lot in the 1920s, and often they were funny in ways that can help Americans of the "70s to see themselves in perspective.

Some readers of this rambling, rather disjointed and sometimes quaintly moralistic study may feel,however, that Ethan Mordden has done more of that specialized reading than is good for him. What emerges in this volume is not so much as account of American history in the '20s, but a running argument with that history. The effect seems, however, to be precisely what Mordden intended; among other clues to this fact, we have the book's subtitle.

For prospective readers, the question is not so much whether this is a history as we use the term today - it is not, though it sometimes resembles the work of those classical historians (Herdootus, Tacitus, Gibbon) to whom history was first an art and only secondarily a strict scholary discipline. The question is whether the reader is looking for the kind of writing that Mordden offers, and the answers are likely to be mixed. He is highly readable but sometimes lacking in focus. There is little depth or breadth of vision, but the work expresses a distinctive personality and a moral concern that is undoubtedly good for us.

The occasional lack of focus may be blamed largely on the author's choice of subject; there was so much happening in the '20s and, despite the lapse of half a century, some of it is still so close to us that it is difficult to see in perspective, particularly when the author has goals beyond the mere orderly presentation of historic fact.

Among the decades of this century (each of which has a fairly well-defined character of its own), the '20s stand out with a special vividness against their rather monochromatic neighbors. They were the age of Rudolph Valentino and Charlws Lindbergh, of Henry Ford, Warren G. Harding and Calvin Coolidge; the decade when moving pictures began to talk and radio was transformed from a hobby into a big business; the age of the flapper and the period that climaxed in the Great Crash. The court-rooms of the period rang with symbolic cases that ranged from the Scopes "monkey" trial to the long agony of Sacco and Vanzetti - plus such added diversions as the Teapot Dome scandals, various spectacular murders and the farcical divorce of millionaire "Daddy" Browning and his childbride, Peaches.

All these themes and a good many more are treated to some extent in "That Jazz," complete with reflections on What It All Means and how it helped push us toward where we are today: a nation with celebrities rather than heroes, manipulated by big-money interests and mass-connunication media, suffering large gaps in our political leadership. Orchestrating so much material for a reasonably unifield effect is no small achievement. The trouble is that readers may feel it is orchestrated - that the information has been sought out and explored not for its own sake but as a part of the author's design, as a prelude to his climactic summarizing statement: "It had been an insufficiently moral time, an Era of New Prerogatives but not of New Responsibilities nor of many New Achievements, not only on the field of state but in virtually every arena except that of the arts. It was that worst of times, one without purpose." Take that, '20s.

Ultimately (the difference emerges when you compare his approach to that of William Manchester in "The Glory and the Dream," which picks up approximately where "That Jazz" leaves off), the difficulty is that Morden treats a whole decade of history not as something that happened and deserves detailed inspection, but as a media event - colorful material to be manipulated for his own purposes.

Mordden is considerably more successful in another recently published work of history, which is addressed to a somewhat more specialized readership: Opera in the Twentieth Century: Sacred, Profane, Godot" (Oxford, $12.95)

Part of his relative success is based on the fact that his field of study is circumscribed and subject to fairly clear-cut modes and standards of judgement. In addition, he shows (as probably he should) more respect for his subject-matter. But above all, it makes perfect sense to treat opera as a media event; that's exactly what it is. The '20s may have been that, but they were also a bit more.