GARRISON KEILLOR knows Minnesotans and he knows radio.So it was just a matter of time before this author of four bestsellers would tackle both subjects in one book. The action of "WLT: A Radio Romance" revolves around a fictitious Minneapolis radio station, WLT (With Lettuce and Tomato). Keillor, who hosted Minnesota Public Radio's "A Prairie Home Companion" from 1974 to 1987, chronicles the birth-to-death cycle of WLT, spending most of his time tarnishing radio's Golden Age image. He intersperses a cast of fictitious radio characters with real-life radio personalities of the period. Beginning with WLT's first broadcast in the mid-1920s, Keillor leads the reader through a 1930s and '40s nostalgic time warp, complete with a cameo appearance by CBS tycoon William S. Paley and mentions of Amos & Andy and Fibber McGee & Molly. But his "warts and all" depiction of everyone - from the crude child star Little Becky to the simple-minded Dad Benson - is warts heavy. In WLT's "Friendly Neighbor" station, everyone is excruciatingly human - and never likable. As Keillor intertwines the radio and personal lives of this cast of antiheroes, the reader is occasionally reminded of the darker moments of his "Lake Wobegon Days" monologue; especially as the show neared its end in 1987. Keillor has concocted a loose, two-part plot in an attempt to tie together vignettes of the book's dozen or so most frequently mentioned characters. There are the Soderbjerg brothers, Ray and Roy, who start WLT in 1926 to rescue their failing restaurant, and the various fictitious Twin Cities radio personalities who work for their station during the next quarter century. And then there is Francis With, the young man from North Dakota who joins the station, changes his name to Frank White, and becomes Ray's fair-haired boy, walking in and out of the studios - and lives - of the book's other characters. WLT concludes with an epilogue that jumps to 1991 - 40 years from the end of Keillor's penultimate chapter, "The End." In this final section, Keillor recaps the post-WLT career of White, who jumped from WLT as the radio ship sank. White landed in TV, where he took his glib personality and questionable talent to become a fictionalized Walter Cronkite. Keillor's own program and his four previous books were mostly rated. However, most of the WLT characters act out their lives in -rated scenes. Instead of finding humorous anecdotes about Lutheran ministers - the sort of Minnesota stories he has often woven so well in past works - readers will find steamy scenarios and "adult" content. Throughout WLT, Keillor's deep, droll radio voice is omnipresent. The cadence, the words, the naming of Minnesota towns such as Roseau, International Falls, and Grand Marais all remind readers of his broadcasts. Indeed, in an abridged audio version of WLT (six hours on four cassettes) also released this month, one can only imagine which of the book's sexually explicit passages might be included. To paraphrase Sen. Ernest Hollings in his vice-presidential debate with Dan Quayle in 1988, Keillor is no Mark Twain. Still, during the past five years, comparisons can be made. Both writers deftly decipher human nature with dry, Midwestern wit. Acid can flow from their pens. And the endings of both authors' books sometimes are forced and unnatural. Except for the last years of his life, however, Twain was not overly negative. Unfortunately, there is a bitterness in Keillor's writing - or at least in his latest book. It's a bitterness that Minnesotans detected in his radio programs in the mid-1980s and that led in part to his fall from his pedestal as the favorite son of the "Land of 10,000 Lakes." IT'S one thing to debunk a myth - in this case, the myth that all was rosy in radio during its Golden Age. Keillor, though, decapitates, dissects, and disembowels the myth, leaving the reader to wonder whether there was anything at all golden about it. But radio in the '30s and '40s did provide inexpensive entertainment during the Great Depression, it did present an array of talented comedians to take people's minds off of their pre-World War II troubles, and it did inform people of international news that helped put listeners' problems in perspective. This side of the record never receives air time in "WLT."