Cheryl Strayed is already something of a phenomenon in the US, where her hiking memoir, Wild, held the No 1 spot on the New York Times bestseller list for seven consecutive weeks last year. Oprah Winfrey restarted her book club on the strength of the book and Reese Witherspoon optioned it for film before it was even published. Nick Hornby is writing the screenplay.

Before all this furore, though, Strayed was quietly beavering away as agony aunt "Dear Sugar" on the literary website The Rumpus. Her (now discontinued) column went viral after a reader asked a simple question: "Dear Sugar, WTF? WTF? WTF? I'm asking this question as it applies to everything every day." Sugar's reply was extraordinary. Breezily, matter of factly, and without a trace of self-pity, she wrote back referencing the fact that she was sexually abused by her grandfather as a child. Her message? Get over yourself. "Ask better questions. The fuck is your life. Answer it."

If this is already making you bristle, you will not like this book. In fact readers of a particularly British sensibility will hate it with a passion, with its casual therapy-speak and frequent mentions of withholding and enabling. As for me, I couldn't bring myself to stop reading it. It made me laugh out loud and it made me gasp with disbelief. However great the writing – and the writing is addictively, breathtakingly great – you do have the sense of entering a quasi-literary version of The Jeremy Kyle Show. But I didn't have too much of a problem with that.

There's a lot of compassion and hard-won wisdom here and a lot of straight talking to people who are trying to convince themselves to do very stupid things (like hold out for a man who is evidently in love with another woman or spend a "happy family Christmas" with a drug-addicted brother who has already tried to attack his parents).

This is Sugar talking to a 26-year-old novelist with writer's block who is crippled with envy: "We get the work done on the ground level. And the kindest thing I can do for you is to tell you to get your ass on the floor." To a woman destroyed by her husband's infidelity: "A terrible thing happened to you, but you mustn't let it define your life."

The majority of her replies could be prefaced with the words "I've been there". In that sense this collection also reads like a memoir: although all the replies were written anonymously (Strayed "came out" as Sugar last year) we get a very real sense of a complicated, painful life behind the advice.

Sugar is a great agony aunt because she never judges anyone. She knows she could just as easily be them. As she says to a father of four addicted to painkillers whose business is falling apart: "I've never been in a humiliating situation when I wasn't shocked by all the 'normal' people who were also in the very same humiliating position. We're horny, ass-saving, ego-driven drug fiends, among other, more noble things."

This book is also a rather wonderful portrait of digital, anonymous America. Sugar used to receive thousands of emails: she simply replied to those that interested her the most. So the questions are always fascinatingly worded and often strange. They reveal a world full of self-obsessed, fractured, self-deprecating people who just want to do the right thing. Maybe that's a very American way to be. Sugar would argue it's just a definition of humankind: "The story of human intimacy is one of constantly allowing ourselves to see those we love most deeply in a new, more fractured light. Look hard. Risk that."