“Improv Nation” is at its more assured in the pre- and early-history of the form, when the number of improvisers was small enough that telling the story of the art through a collection of personalities is more manageable. Wasson presents a textured portrait of Viola Spolin, the idealistic teacher who developed improvisational games in the 1940s that actors and comedians would study for generations. She saw her work as a rehearsal tool, one that her son Paul Sills brought to the Compass in the next decade, which drew an incredible collection of talent to topical sketch work and freewheeling improvisation.

The early relationship of Nichols and May is laid out here like a romantic comedy. When Wasson reports that May slept with Spolin’s son Paul, he adds: “Nichols pretended not to care.” When the Compass moved to St. Louis, Wasson gravitates toward another triangle, this time involving the actor and teacher Del Close and Nichols, clashing not just over May, but also over the future of improv.

In a dispute that would replay itself over the decades, Nichols thought improvisation had limitations in how deeply it could explore psychology and character, while Close believed in its potential as an art form and as an end in itself. Nichols and May moved to New York, where they made a commercial hit out of their double act, performing still-funny sketches followed by an improvised scene. But it was Close who became the ultimate guru, teaching the future stars of comedy and anticipating where the form would go.

Some of the best parts of the book are its explorations of overlooked pioneers, like Wasson’s superb reporting on Valri Bromfield, a brilliant and largely forgotten comic who started on a comedy team with Dan Aykroyd. She appeared on the first episode of “Saturday Night Live” and was a regular on David Letterman’s short-lived morning show.

As the narrative moves toward the last few decades, these more obscure stories get pushed aside to make room for the familiar ones. The focus on the film careers of “Saturday Night Live” stars raises a broader question: What is improvisation anyway? Or more to the point, what isn’t?

Early offspring of the Compass, like the first New York troupe The Premise, which opened in Greenwich Village in 1960, leaned on rehearsed material while billing itself as an “improvisational revue,” which earned skepticism from the press, including a review from The Village Voice calling it a “ruse.”

Wasson casts a wide net, discussing “Ghostbusters” and “The Colbert Report” alongside the brilliant current long-form team T.J. and Dave, but at times, he becomes more exclusive, declaring, for instance, that commedia dell’arte is not part of the history. He rightly emphasizes that long-form improvisation is a distinctly American form, but overstates the case — you see this even in his title — ignoring major figures and traditions in other countries. What makes this more than a minor omission is that the globalization of improv would be powerful evidence to support the book’s effusive arguments. After all, the late-night host Seth Meyers got his start in an improv theater in Amsterdam, and a new club in London, the Bill Murray, offers improv classes run by Second City. To do justice to the impact of improv comedy, you need a wider lens, one that explores the increasing importance of improv theaters in the comedy ecosystem, the various schools of pedagogy and how the principles of improvisation have infiltrated the business world, traditional acting and popular culture.

The old debate about whether or not improv comedy is an art in itself or a means to create work now seems quaint. It’s bigger than art. What began on the South Side of Chicago more than six decades ago has become not just an art or job but, for some, a worldview. Tina Fey described it that way in her best-selling book “Bossypants.” She promised that the rules of improvisation would “change your life and reduce belly fat,” before admitting that the part about belly fat wasn’t true.