Zinoman, who is this newspaper’s comedy reviewer, seems to have all those dimensions at his fingertips. However, he’s too lively a writer to get bogged down in show-offy theorizing. His big-picture commentary is so compressed and fluid that you often scarcely notice how casually he’s able to switch from micro to macro and back inside a single paragraph. As celebrity biographers go, he’s humane but not easily fooled (Zinoman interviewed Letterman, as well as many others associated with his shows). As a critic, he’s especially sharp and engaging when he’s breaking down Letterman’s trademark predilections, from the goofball love of peculiar locutions and nonsensically stressed clichés that evoked a sort of demented neo-Americana to how unerringly “his comedic instinct was to mock and belittle whatever world he inhabited.”

The latter impulse stayed constant even when the primary world Letterman inhabited on “Late Show” was his own curmudgeonly, dissatisfied brain. But it also predated his fame. As early as middle school, he was equating self-expression with media constructs, enlisting a classmate to stage sham talk shows on a mock set in the friend’s basement. (Shades of Rupert Pupkin in “The King of Comedy.”) By the time he got to college at Ball State University in Indiana, though, his invariable M.O. as a campus-radio disc jockey was to burlesque the conventions and/or pretensions of whatever format he was assigned, including introducing Debussy’s “Clair de Lune” with a breezy “You know the de Lune sisters. There was Claire, there was Mabel.” That particular bit of irreverence promptly got him canned, though not for long.

Then followed a few scruffy years on local TV, including his celebrated stint as a smart-aleck weatherman, before he nerved himself to head to Los Angeles in 1975. Once there, he did stand-up at the Comedy Store, appeared on game shows, shot a failed pilot for one of his own, and spent a few unhappy weeks on Mary Tyler Moore’s flop variety series, “Mary.” Then he got booked on “The Tonight Show” at a time when Johnny Carson’s benediction was pure gold. Letterman’s own “The David Letterman Show” debuted on NBC in 1980, and though it didn’t last long — the morning time slot was all wrong for him — it provided the template for everything he was to do in late night.

By then, he’d already found an invaluable accomplice: the writer and comedian Merrill Markoe, his girlfriend for the better part of a decade and the decisive creative influence on both “The David Letterman Show” and “Late Night.” (“Without her, you and I wouldn’t be sitting here,” Letterman told an interviewer after their breakup.) But Zinoman gives almost equal credit to the veteran director Hal Gurnee, who grasped right away that television wasn’t only Letterman’s medium; it was his subject. Now that “meta” is the conceptual given of so much TV comedy, it’s easy to forget how inventive Gurnee’s visual equivalents of putting everything inside air quotes actually were. The same is true of Letterman’s longtime musical sidekick, Paul Shaffer, whose parody version of Vegas smarm defined “Late Night” as pure showbiz and pure anti-showbiz simultaneously.

Zinoman isn’t wrong to spend over twice as many pages on Letterman’s “Late Night” years — lasting just over a decade, from 1982 to 1993 — as his much longer stint hosting “Late Show” (1993-2015). After all, “Late Night” arguably changed the face of television, while its much plusher CBS sequel mostly just changed the face of its host. Not always for the better, either; as Zinoman observes, “Letterman increasingly played the horny creep” with female guests as he aged. He got a comeuppance of sorts when a 2009 blackmail threat forced him to pre-emptively admit he’d cheated on his wife with more than one of the show’s staffers. By then, however, he was such a famously odd duck that Steve Martin congratulated him on the air for humanizing himself.

Interestingly, even on “Late Night,” Letterman had much less investment in staying quirkily cutting-edge than his writers did, particularly once alumni of The Harvard Lampoon and “Saturday Night Live” began dominating the writers’ room and pushing for more audacity. Emulating and eventually succeeding Johnny Carson remained his goal, one he fell short of once Jay Leno inherited “The Tonight Show” instead. Despite Zinoman’s Generation X birth date, he knows his television history well enough to argue that Letterman was as much a throwback to midcentury TV as a postmodern iconoclast, modulating from Steve Allen’s style (waggish, superior and happily ridiculous) to Jack Paar’s (neurotic, confessional, tormented). Yet Letterman’s true originality may be that only a Midwestern boomer who was every bit as easily unnerved as he was nervy could have combined the two. Or, at any rate, turned them into chapters of the same generational odyssey.