The dizzying fecundity of the entrepreneurial mind is in on full display toward the end of Tracy Kidder's "A Truck Full of Money: One Man's Quest to Recover from Great Success." Kidder's latest book is about Paul English, co-founder of the travel search site Kayak. The chapter in question concerns--of all things--URLs.

English is a man who does not rest easy. Ideas vie with demons to populate his brain at night. "Often he woke to find a new idea waiting in the doorstep of his consciousness," writes Kidder. For years, English captured his ideas--dream-born and otherwise--in a list of domain names registered to him. By 2013, the list had reached 158 entries.

Some domains became products and businesses, such as Roadwars, a safe-driving game; and Boston Light, a developer of ecommerce software. (Intuit acquired Boston Light, one of five technology companies co-founded by English, in 1999.) Most, though, either failed to gestate or quickly withered.

Take Purplewatch, which would sell specially made all-purple Swatches for $10,000, with profits going to a charity. English imagined country-club habitues wielding the wrist wear as a socially conscious status symbol: "I'm rich too, motherfucker. Except my ten grand went to an orphanage."

High-minded or irreverent. Transformational or trivial. For seven pages, English's brain spills and spills and spills. "The mind, after all, is an incubator," writes Kidder.

Kidder knows from tech visionaries. His second book, which won a Pulitzer Prize in 1981, was "The Soul of a New Machine," about a team of engineers at Data General developing a minicomputer. Thirty-five years later, elementary school students can reel off the specs for the latest iPhone, so layman-izing this subject matter is less of a challenge.

In fact the story suffers, at first, from our familiarity with this world. Kayak's headquarters are startup boilerplate: "no private offices; conference rooms with glass walls; heating ducts and pipes left exposed in the high ceilings." Coders are fashion-challenged, with many "on the spectrum." English peppers his speech with terms like "awesome," "pixie dust," and "rock star."

But if Kayak sounds like a Silicon Valley cliché--albeit one based in Concord, Massachusetts--English is not. Yes, he is brash and brilliant and laps up risk like a thirsty puppy. His ambitions are Brobdingnagian. ("We'll do an Amazon killer. Probably first a Fidelity one and later a PayPal one.") And his entrepreneurial chops are obvious.

The book's title comes from a comment made by Bill O'Donnell, one of several English sympathizers who follow the serial entrepreneur from business to business. "Someday this boy's going to get hit by a truck full of money," says O'Donnell, "and I'm going to be standing beside him."

One thing that sets "Truck" apart from comparable tales--beyond Kidder's narrative prowess--is that English's chief nemesis is not a wily competitor or myopic VC. Rather, it is his own brain: the same brain that leaps effortlessly from A to C and churns ideas at warp speed. English is bipolar, and Kidder's descriptions of how he started, scaled, and led businesses while surfing the swells of mania and the troughs of depression are riveting.

At the apex of his highs, "he felt 'on fire,'" writes Kidder. "Everything seemed possible for him and the success of every new venture assured." Other times he is beset by "gloom and lassitude and fearfulness, at time panic, and it felt like something deep and wide moving inside him, like a chemical flood." At night English would leave his bed and crawl across the floor to the window, where he would huddle, waiting for the sun.

The temptation is strong to defy Susan Sontag and treat illness as metaphor. After all, healthy people experience company founding as a series of outsize highs and lows. ("If someone invented a drug that normal people could take to feel like I feel this morning, that inventor would be a billionaire," writes English in an email.) But English succeeds in spite of his disorder, not because he periodically embodies some entrepreneurial ideal of energy and creativity. Passages about his illness evoke pity in the reader. They provide perspective.

From Commodores to Uber

English, the child, was very much the father of the man. One of seven children growing up in a family where money and parental affection were stretched thin, he was a fighter and a rule-breaker. He was also a congenital geek. In the wonderful anecdote that opens the book, a 12-year-old English engineered the theft of a teacher's password to gain access to the school's lone computer. When five years later his mother brought home a Commodore computer, he treated it like the restoration of a missing limb.

As a professional coder, English was brilliant. As a manager, he was a brilliant coder. Tech founders will relate.

Kidder weaves among scenes of English's youth, his early companies, his time at Kayak, and his more recent exploits--notably an incubator-cum-nightclub called Blade that ultimately morphed into his latest travel venture, Lola. Themes recur throughout. There is money, to which--for a kid who grew up poor--English seems largely indifferent. He is obsessional that those around him must be richly rewarded. He practically thrums at the prospect of spending a fortune on a custom couch, a tricked out truck, a lighthouse.

On a more serious note, Kidder describes at length English's touching philanthropic education at the hands of Tom White, co-founder of Partners in Health, the medical charity active in Haiti. "In a recurring fantasy," writes Kidder, "Paul was sitting in Tom's Cambridge apartment, drinking gin and tonics, and he was saying, 'I made the big score, Tom. Let's figure out how to give it away.'"

There are also the loyal friends who follow English from company to company: colleagues more than confidantes, but--to wax Jerry McGuire--they complete him. Of those, the most endearing is Karl Berry, a former college classmate and talented programmer who plays Eeyore to English's Tigger. "I hope I can convince you" of the immorality of software patents, Berry writes in a typical email. "If I can't convince you, I despair."

As the book progresses, the ambitions grow bigger, the plans vaguer, the investors more involved. With English's over-the-top scheme for Blade, the line between vision and mania appears all but erased. "He was trying to make a commercial office that would be a work of art, a place where many of the things he loved could be assembled, a fusion of his multiple enthusiasms, for teams and music, fine art and first-rate software, parties and hard work, old friends and new," writes Kidder. "He couldn't stop himself."

The book's end finds English on a more or less even keel. He has finally broken all ties with Kayak, sold to Priceline in 2012 for $1.8 billion. He is elbows-deep in Lola, a neat concept that makes human travel agents available online. And to research the effect of customer ratings, he is working part-time as an Uber driver. "Some nights he made as much as $50," writes Kidder.

Paul English is that rare thing: a successful entrepreneur who is very "what you see is what you get." No polish and no packaging. You cannot be him. Probably, you don't want to be him. But to know him, surely, is a wonderful thing.