A mash note to the current king of the late-night talk shows. For almost five years now, Jay Leno has hosted the Tonight Show, one of American entertainment's great institutions. To get there, he served a hard apprenticeship on the comedy club circuit, touring ceaselessly, playing almost any club that asked. He also worked endlessly at his craft, perfecting his timing, honing his jokes. But despite paying his dues and his growing national visibility, Leno found that succeeding Johnny Carson was an unexpectedly difficult challenge. Critics thought Leno's version of the show was stiff, awkward, and unfocused. And audiences seemed to agree: Ratings sank, and David Letterman was very publicly considered as a replacement. Leno eventually won this battle, and Letterman moved to his own rival show, on which he trounced Leno regularly. Then Leno revamped the show, beginning a long battle back to ratings dominance. These maneuvers and campaigns were recently and expertly covered by Bill Carter in his thorough and entertaining book The Late Shift (not reviewed). Apart from a strong pro-Leno slant, New Yorkbased Walker rarely offers more than a rehash of Carter's work, as well as the sort of broad details and shopworn anecdotes that seem gleaned from celeb interviews in glossy mags. Walker does offer extensive examples of Leno's observational and topical humor. (Leno explained his brand of humor by saying, ``I just travel the country and identify the absurd.'') He also details in some depth Leno's comic process, how he builds and structures his jokes, always seeking ``to get to the punchline as quickly as possible.'' Walker, unfortunately, does not share his subject's unerring concision. He is given to frequent repetitions, tangled syntax, and much belaboring of the obvious. This is more a silhouette than a full-fledged biographical portrait, and no competition for Leno's own current bestseller.