SATURDAY NIGHT: A Backstage History of 'Saturday Night Live'. By Doug Hill and Jeff Weingrad. Morrow/Beech Tree. 510 pp. $17.95.

FOR A FEW brief, glorious years, from its first season in 1975-76, until sometime in the late 1970s or early 1980s (depending upon one's personal point of view), when the show began to lose its creative edge along with many of its most talented players and its creator Lorne Michaels, and gradually degenerated (save for occasional flashes of brilliance) into a lame parody of its former greatness, NBC-TV's Saturday Night Live succeeded in reshaping the humor of an entire era.

SNL's unruly and iconoclastic cast of writers and actors was also responsible for almost singlehandedly -- if only temporarily -- restoring to network television two ingredients that had long been msing: relevance and spontaneity.

Who among us, after all, having witnessed John Belushi's Samurai sketches, Gilda Radner's Roseanne Roseannadanna, or Bill Murray's unctuous Nick the Lounge Singer could ever forget them? Who, even now, can recall without the least gleeful spasm, the Coneheads, the Killer Bees, the Nerds, or the other bizarre and largely unprecedented comic creations of the Not Ready for Prime Time Players? What Saturday Night Live's innovative and improvisational -- if only occasionally cruel or tasteless -- presentations added up to was simply the most provocative, irreverent and socially-informed live comedy ever unleashed on network TV, either before or since.

Such an overwhelming estimation of SNL's sprawling, pervasive influence and originality is obviously shared by veteran TV journalists Jeff Weingrad and Doug Hill (the latter a staff writer for TV Guide.) The two of them have written a rather massive (nearly 500 pages) and impeccably researched book about the show whose impact on popular culture, they rather convincingly assert, "has been equaled only by the likes of Steven Spielberg and George Lucas."

And a splendidly informative and entertaining book this is too. A Backstage History of Saturday Night Live succeeds admirably from a multiplicity of perspectives: critical, historical, sociological. . . Hill and Weingrad even meticulously trace the show's genesis and sometimes precarious survival amid the treacherously conservative jingle of NBC's corporate politics.

To their credit, the authors remain surprisingly even-handed and calmly investigatory throughout the book. Their narrative seldom, if ever, bogs down under the plentitude of details, statistics and Nielsen-rating breakdowns which they furnish. Nor does it get swept away by the potential sensationalism of the seemingly endless behind-the-scenes antics, intrigues and debauches which they chronicle. The result is a text that is painstakingly comprehensive, yet immensely readable -- sort of like Indecent Exposure, only without the accompanying scandal.

Hill and Weingrad do make it clear, though, that what looked like great fun to the huge TV audience was actually, from an insider's perspective, more like a special circle of Dante's Hell reserved for young and desperately ambitious comedians. From the beginning, the pressure-cooker intensity of competing for precious on-camera time, combined with the massive stress of delivering the goods under the harsh, unforgiving eye of the live cameras there in the cramped confines of NBC Studio 8H undermined all but the slimmest potential for genuine off-camera camaraderie or lightheartedness.

Instead, the backstage atmosphere quickly came to resemble that of an underfed and overcroweded piranha tank. Sexism and racism were just a few of the milder charges the various cast members hurled at each other. (There was even a pre-show fistfight between Bill Murray and Chevy Chase, the latter of whom was the show's first major star and -- partly as a result of this -- the one most hated by the rest of the staff.)

Practically every SNL actor and writer seemed to cave under this pressure, each in his or her own way. ("A hornet's nest of drug problems," is how one insider described the atmosphere in the show's offices on the NBC building's 17th floor.) Thus we have accounts of: Garrett Morris' free-basing paranoid hallucinations, Laraine Newman's anorexia (she was down to 80 pounds at one point), Gilda Radner's bulimia (she once grimly boasted of having thrown up in every toilet in Rockefeller Center), and Chevy Chase's egomania.

ASIDE FROM being an exhaustive history of SNL, Saturday Night is also something which the authors, in their evenhandedness, probably didn't intend it to be: a casual indictment of the overriding mentality of network television and the men who populate its highest corporate echelo With far too few exceptions, the members of NBC-TV's corporate brass emerge as a tawdry lot, as myopic, instinctively repressive, and (in a few cases) as corrupt as a dark-suited Soviet presidium. Many of these executives remained (even after SNL had swept the 1976 Emmy Awards, become a huge Nielsen-winner, and -- hence -- immensely profitable to the network) openly resentful and hostile toward the show whose humor and originality was clearly beyond their grasp.

Just as meticulously, Hill and Weingrad trace SNL's gradual creative decline, right up through its more recent, lackluster (and in some cases) downright disastrous seasons, during which the show has survived in name only, long after its original innovative spirit died on the vine. (The authors point out that even a travesty was made of the show's title during the 1984-85 season, during which as much as 40 percent of the supposedly live weekly program was actually pre-taped.)

But amid the grimness and mild pessimism of these final chapters, there is a certain spirit of optimism that nonetheless prevails. I'm sure I am not the only reader who will come away with the feeling that, whatever pain was endured by those who manned the trenches at Saturday Night Live, it was certainly worth the gain, at least, in sheer artistic terms.

For there is an obvious ray of hope to be found in reliving the brilliant, if sometimes anguished, comic guerrilla insurgency that the SNL crew waged against network television's institutionalized blandness. The hope lies in the fact that such a glorious battle, having once been successfully waged, might someday be waged again.