When a smart café opened in his neighbourhood in New York, where he used to live, Buddhist monk and bestselling author Haemin Sunim went along to sample the delicious-looking cake. Hearing the prices, however, he balked and ordered just tea instead – but that cake stayed in his mind all afternoon. The next day, he was still thinking about it, and the day after that until, finally, he had to go back and treat himself. The verdict? “It was delicious but not extraordinarily delicious,” he writes in his new book, Love for Imperfect Things. “This must be the kind of feeling people have after winning the Nobel prize or becoming president.”

This small, wry slice of everyday life – and let-down – is one of many in Sunim’s latest self-care tome, the follow-up to his first book, the wildly successful Things You Can See Only When You Slow Down, published in 2012.

Sunim, 45, has been dubbed a “megamonk”. He has the kind of celebrity status that can turn a short stroll through his native Seoul, South Korea, into an embarrassing round of selfie requests. With over 1m followers on Twitter and Facebook, his first book was a quiet call for calm in our insanely busy world. Translated into 30 languages, it sold in the millions. Sunim offers bite-size Buddhism – mindfulness for the modern age – mainly through short anecdotes or sparse, haiku-like verse. His writing is gentle and meandering, and often so simple as to seem quite obvious. Haemin Sunim (which means “spontaneous wisdom”) gives insight and advice we might know already, but still fail to heed.

The central message in Love for Imperfect Things is the importance of loving and accepting our imperfect selves, our imperfect world – and imperfect cake – the way a mother loves her child, no matter what. The book examines the many ways expectations fall short. Our parents don’t understand us. Our children don’t appreciate us. We didn’t nail that “dream job” or, if we did, it hasn’t delivered what we dreamed it would.

His celebrity status can turn a stroll into a round of selfie requests

“That ‘disappointment’ is a problem of humanity, not just in the west,” says Sunim, speaking to me from New York, where he has come to meet his spiritual teacher. “One of the reasons I became interested in our relationship with imperfection was reading Sengchan, the sixth-century Chinese Zen master, who said: ‘True freedom is being without anxiety about imperfection’. I’d never thought of ‘freedom’ in those terms and I wanted to expand that idea.

“Often, one of the hurdles for loving and accepting yourself is the idea of how you should be, how your life should be. But that picture of perfection resides only in your imagination,” he continues. “I’m not saying you shouldn’t try to be good at something or have a good life, but if you’re dissatisfied and want to change things all the time, you are going to find yourself very unhappy. So the trick is, how can you make peace with what is? That’s the recipe for happiness.”

Love for Imperfect Things is packed with random advice on how to achieve this, and addresses the worries and disappointments that regularly arise during Sunim’s public talks and Q&As. If you’re worried because your spouse or child has gained weight, for example, Sunim suggests setting an example by looking after your own health with a varied diet and plenty of exercise. Pointing out a loved one’s faults is more likely to hurt them than make them change, he says. To those searching for the perfect soulmate, Sunim suggests there’s “no such thing… When you work hard to make your relationship work and stay together for a long time, then you each become the person you were meant to be with.”

The book’s glimpses into Sunim’s own life as a Buddhist monk are especially enjoyable. The cake incident is one example. In another, Sunim remembers a meditation retreat where an older monk directed him to sweep the steps. Since he was already volunteering diligently in the kitchen, Sunim spent what he later saw was an embarrassing amount of time seething about the extra task, which, in reality, took less than five minutes to complete. From this, he learned to be more mindful of negative thought patterns triggered by very minor issues.

Sunim also describes breakthroughs in his own family relationships. One concerns his father, who used to irritate him by always putting himself last and insisting he was “unimportant”. For this reason, when he rapidly, inexplicably lost weight, he even put off seeing a doctor.

“We get the most annoyed by the people we are closest to,” says Sunim. “What helps – and what helped me – is not wishing they were different or telling them to change but instead trying to understand them better. I made the effort to talk to my father and ask him questions, not in relation to me but as a separate human being.

“What I found was very surprising,” he continues. “I saw a man whose own father hadn’t shown him a lot of love and affirmation. His father had valued his oldest son – and my father, the second son, had internalised that. He saw himself as unimportant. I think understanding your parents as separate to you is a sign of maturity.” His father’s health has since recovered and in the book Sunim thanks him for raising a son with high self-esteem and a positive outlook.

Sunim’s father has enjoyed the book – both parents are extremely proud of their son, though surprised by his “career path”. As a student, he left South Korea to study film at the University of California, before switching to religious studies. “I thought film would be a fun and interesting process, but it wasn’t,” he says. “Making one short film was very time-consuming, such a group effort with so many elements out of my hands. I instantly realised that was not my path. I’d always been very interested in religion, so studying that was very natural for me.” He took a master’s at Harvard, a doctorate at Princeton and then taught in an American college.

‘Sometimes I need to read my own books for my own advice’

At 25, he decided to become a monk. “I was more interested in personal, spiritual experience than being a scholar,” he says. “As an academic, you study someone else’s work not so much to inspire or deepen your practice but to prove points – it’s logical, scientific. With spiritual practice, it’s more personal; it often transcends language.”

Things You Can See Only When You Slow Down was published first in South Korea, where it spent 41 weeks at number one. I wonder if its success surprised him – after all, the mindfulness message has been heard before. “I think it’s because I’m writing to help my readers,” says Sunim. “I’m not doing it as an artist, or to give myself expression. People can see that and, traditionally in Korea, people turn to Buddhist monks for advice. I’m part of that.”

Sunim admits the success brought pressures. “I didn’t expect the book to become so popular, so I didn’t know what to do with it,” he says. “You get stopped in restaurants, or in a coffee shop, and asked for a photo. I try to be as gracious as I can, but it’s a little bit awkward for me; though, overall, I’m grateful.”

Writing is just one way Sunim spreads his message. In South Korea he has opened – and funded – two healing centres he calls Schools for Broken Hearts. Anyone navigating difficult times – divorce, illness, loneliness, family breakdown – can come, connect, and access practical help from teachers and psychologists. “Whether you’re Christian, Buddhist, Islam… this is for everyone,” says Sunim. “That’s what my life revolves around now.”

With 50 part-time teachers and 12 full-time employees, it sounds quite an undertaking. In fact, Sunim’s schedule does seem rather full-on for a monk famous for urging us all to slow down. He was in New York when we speak, he’s in London this month to promote his book, then he will be back in South Korea, where he combines work at his schools with meditation retreats, plus public Q&As across the country once or twice a day. Add to this a meditation app which he hopes to launch later this year. Is he packing a lot in?

“I have reduced my engagement with social media,” says Sunim. “I spend less time on Twitter, Facebook, Instagram, things like that.

“Sometimes I need to read my own books for my own advice,” he admits. “Just because I said it doesn’t mean I’ve already embodied it! For me, as well as the readers, my books can be a gentle reminder of the truth we probably already know.”

Love for Imperfect Things is published by Penguin at £9.99. Buy it for £7.49 at guardianbookshop.com