Where surfing took Finnegan was around the world. The post-“Gidget” boom of which he was a part made the sport an iconic, global phenomenon, but it also caused breaks from Malibu to Pipeline to become miserably crowded. As a consequence of this ruination, and the era’s broader idealism, adventurous surfers like Finnegan went in search of solitude and the perfect, prelapsarian wave. Along the trail, he and his travel companion found themselves carrying the weight of more than just their backpacks and surfboards. On the one hand, “chasing waves in a dedicated way was . . . dynamic and ascetic, radical in its rejection of the values of duty and conventional achievement.” On the other, “being rich white Americans in dirt-poor places where many people, especially the young, yearned openly for the life, the comforts, the very opportunities that we, at least for the seemingly endless moment, had turned our backs on — well, it would simply never be O.K. In an ­inescapable way, we sucked, and we knew it.” In other words, once it emerges from the ­adolescent-rebellion stage of its development, surfing presents itself as a problematic passion, and it is one of this book’s many great strengths that it unflinchingly addresses the various forms this problem takes as Finnegan grows up, commits to a career as a journalist and has a family.

Yet find the perfect, empty wave Finnegan did, back in 1978, off a tiny island in Fiji. The moment of revelation is the surfing equivalent of Keats’s “On First Looking Into Chapman’s Homer”: “We turned and trained our binoculars on the tiny island across the channel. We were looking straight into the wave. . . . It was a long, tapering — a very long, very precisely tapering — left. The walls were dark gray against a pale gray sea. This was it. The lineup had an unearthly symmetry. Breaking waves peeled so evenly that they looked like still photographs. . . . This was it. Staring through the binoculars, I forgot to breathe for entire six-wave sets. This, by God, was it.”

Now among the most renowned in the world, the Fijian break is one of perhaps a handful in its class that Finnegan has intimate, masterly knowledge of. Indeed, if the book has a flaw, it lies in the envy helplessly induced in the armchair surf-­traveler by so many lusty affairs with waves that are the supermodels of the surf world. Still, Finnegan considerately shows himself paying the price of admission in a few near drownings, and these are among the most electrifying moments in the book.

There are too many breathtaking, original things in “Barbarian Days” to do more than mention here — observations about surfing that have simply never been made before, or certainly never so well: the postsurf moods of “pleasant melancholy” or “mild elation”; the “charged and wild inclination to weep” that comes in the wake of unusually intense rides or ­wipeouts; the unlikely facial expressions actually worn by a surfer in the act of riding a demanding wave; and ­visionary descriptions of oceanic beauty occasionally met with in surfing but seldom done justice: “It was midday, and the straight-overhead sun rendered the water invisible. It was as if we were suspended above the reef, floating on a cushion of nothing . . . . Approaching waves were like optical illusions. . . . And when I caught one and stood up, it disappeared. I was flying down the line, but all I could see was brilliant reef streaming under my feet.”

But a particularly remarkable feature of “Barbarian Days” is the generous yet unsparing portraits of competitive surf friendships that make up a major share of the narrative. As Finnegan writes: “Surfing is a secret garden, not easily entered. My memory of learning a spot, of coming to know and understand a wave, is usually inseparable from the friend with whom I tried to climb its walls.”