When Garrison Keillor sings on "A Prairie Home Companion," he often reins in his sonorous timbre to playful ditties or respectful harmonizing with legendary musicians. Even in its contained state, however, Keillor's distinctive singing voice hints at unplumbed depths that cry out for a more substantial expression. Perhaps he could have combined his undeniable storytelling abilities with his amazing voice to become a haunting singer/songwriter like Tom Waits or Leonard Cohen. But, alas, there is no "Famous Blue Raincoat" written by Keillor, only odes to ketchup and rhubarb pie.

Similarly, Keillor, an outspoken and active champion of great literature, seems content to turn out the literary equivalents of the Powdermilk biscuit jingle: kinda- clever stories that mix retro charm with light irony and whose brief enchantments are soon forgotten.

Keillor's writing, both in his radio work and in his novels, occasionally brushes up against Real Ideas -- the weight of aging, loss of purpose, family and duty as a prison -- suggesting that the man could turn out a really impressive, profound novel if he so chose. The obviousness of Keillor's unexplored depths combined with what is his weakest narrative outing in several years makes his latest frivolous novel, "Guy Noir and the Straight Skinny" too tiresome a read to recommend to any but hard-core Keillor fanatics.

"Guy Noir and the Straight Skinny" is Keillor's first book featuring beleaguered private dick Guy Noir, a satirical Sam Spade-type protagonist based in St. Paul whose "Guy Noir: Private Eye" segments on "PHC" have become an audience favorite. In the scatological plot of "The Straight Skinny," Noir's femme fatale business partner gives him a diet pill that deposits tapeworms in the stomach of the dieter. Noir acts as a guinea pig for the pills and becomes svelte and sexy enough to have young women on both sides of the Mississippi throwing themselves his way. Gangsters and the FDA get wind of what Noir is up to and everyone wants a piece of the action -- and a piece of Noir.

"The Straight Skinny" is much more rambling and scattered than the last few Lake Wobegon novels, with unfunny digressions continually sabotaging any sense of drama or clarity or pace. And like a great singer oversinging a bad song, no pun, alliteration or random ironic coincidence is beneath him. The first Guy Noir novel, unbeholden to the Lake Wobegon oeuvre, provided a perfect opportunity for Keillor to be creative, to try a new style, to be as daring as the writers he celebrates every day on "The Writer's Almanac"; perhaps he could have even written an actual noir novel.

Instead, Keillor's magnificent voice is once again skit-skatting a nonsense song. For those waiting for the time when the 69-year-old multidimensional artist stretches his talents to fruition, "The Straight Skinny" frustratingly augurs that day may never come.

Cherie Parker is a writer in Washington, D.C.