Karen Kilgariff and Georgia Hardstark are not your average agony aunts. For a start there’s the fact that they dispense funny advice and useful self-care tips alongside descriptions of the details of notorious murders. If that mixture of true crime and practical advice sounds surprising then you’ve clearly never listened to Kilgariff and Hardstark’s hit podcast, My Favorite Murder, in which the two women start off each week by discussing an infamous crime before inevitably getting side-tracked into talking about past experiences and current obsessions.

Hugely popular – it pulls in more than 19 million listeners a week and has seen the women sell out venues not only across their native US but also in Australia, Ireland and the UK, where another tour is planned for later this year – My Favorite Murder stands out in an endless sea of true-crime podcasts, largely because of the easy camaraderie between its two hosts, Kilgariff, 49, a writer and former stand-up comedian, and Hardstark, 38, a host on the Cooking Channel and contributor to the popular comedy show Drunk History.

Listening to them shoot the breeze about whatever has caught their fancy that week, from where to go on holiday to what to do if you suddenly find yourself caught up in a cult, is like hanging out at a crowded party with the coolest people in the room, the kind of women who will make you scream with laughter largely by saying something so dark and yet so funny that you can’t quite help yourself.

“I think the appeal is that we are in no way pretending to be or thinking that we are perfect,” says Kilgariff on the phone from Los Angeles. “What we’re saying is that the best thing to do is have a good, happy fulfilling life and if you fuck up then you fuck up and that’s OK.”

The write stuff: onstage at a literary festival in Beverly Hills, California. Photograph: Charley Gallay/Getty Images for Pen America

Hardstark agrees: “We’re living in a period where women are sick of competing against each other and what people like about the podcast is that it’s all about building people up, not tearing each other down.”

My Favorite Murder was born out of a late-night conversation at a Halloween party when both women realised that they shared a similar obsession with lurid true-crime stories. It has proved so successful that they have performed in front of thousands of rapt fans, launched a membership scheme called Fan Cult with an annual cost of $39.99 (£31.50) and now have a website selling a range of merch. This week sees the UK publication of their spin-off memoir, Stay Sexy & Don’t Get Murdered. “Yeah, we really never expected it to get this big,” Kilgariff says. “It’s out of control,” agrees Hardstark.

Yet it’s easy to see just why they have such a huge following. Stay Sexy, an empathetic, insightful and crucially very funny look over the key moments of both women’s lives, covers everything from how to deal with a much-loved parent’s slow slide into dementia to what to do when the life you have turns out not to be the one you imagined, and does so with wit and a weary but hopeful compassion at the state of the world.

“When I was a kid we’d go and visit my grandmother, and my dad is one of eight so they’d all be sitting there updating everyone on the good things but also on the fact that so and so had cancer or had lost their job, and people would make jokes,” says Kilgariff. “I think sometimes it’s only by talking about what’s really going on that you can make it less scary. We all feel anxious at times and acknowledging that only helps.”

As in the podcast, those serious moments nestle up against more light-hearted diversions. Kilgariff explains what life was like as a latchkey kid in the 70s (one step up from trying to survive The Hunger Games). Hardstark delves into what to do if your brother suddenly disappears on a camping holiday (don’t start imagining that he’s been brutally murdered, however much you want to). Both women offer very funny advice on how to party and what to do when your body is telling you the party’s over.

‘The listeners are so clever and funny and weird and cool’: Kilgariff and Hardstark. Photograph: Kevin Winter/Getty Images

“In a lot of ways it’s as much a cry for help as a self-help book,” says Kilgariff, laughing. “For a long time we were languishing in the fantasy that we were going to write a coffee-table book with big writing, funny jokes and minimal effort, but then our editor told us we had to write something more personal and by then we were in too deep to get out.”

From the start they decided that no punches would be pulled either in language or tone, with both hosts prepared to speak honestly about their own issues. Hardstark, who has ADHD and anxiety, had a difficult childhood involving spells in rehab; Kilgariff has had issues with alcohol, drugs and diet pills. Both women are “college drop-outs” who understand what it feels like to struggle to pay the rent.

“We don’t really have the ability to bullshit, so from the start all we could do was talk honestly about our own experiences,” Kilgariff says. “Everyone’s done shit in the past that they’ve regretted, or had moments they wish they’d approached differently. For us, saying that out loud wasn’t a big deal.”

“One of the hardest things in the world is when you can’t connect, or think that no one is listening to you,” Hardstark adds. “I think what works is that we’re not ashamed of saying when things have gone wrong.”

The podcast’s most devoted fans, who describe themselves as “Murderinos”, have credited Kilgariff and Hardstark with helping them deal with their own issues. The duo are very careful to focus on mental health awareness and try their best to ensure that amid the bleak recounting of brutal crimes there is always hope.

“When we started we thought that people would want to talk about the murders, but actually it’s the other stuff they want to talk about,” Hardstark says. “They come up to us and say: ‘I had this stuff happen in my life, too,’ or: ‘You made me more aware of this.’ We have people talk to us about how the podcast has helped them feel better.”

Some of the responses, she says, have blown them away. “It can range from the really heavy to the amazing. We have people who come up and say that listening to the podcast has given them the push they needed to change their studies, and people who say that it’s led to them talking to their mother again – and both those things are equally monumental. It’s incredible that making some jokes and talking about our lives can inspire people in that way.”

Not that the “cult” (as Hardstark and Kilgariff jokingly describe it) is all about self-care and good advice. Murderinos fully embrace both the macabre and the sharp sense of humour, turning up to the arena events with dresses adorned with murder weapons and wearing T-shirts saying: “Toxic Masculinity Ruins the Party Once Again,” a phrase which can also be found emblazoned on greeting cards. “We get some great gifts,” says Kilgariff. “Trophies, very funny things, one woman even made a diorama of us sitting recording the podcast.”

“That was wild,” says Hardstark. “So much of the stuff we get makes us laugh out loud. The listeners are so clever and funny and weird and cool.”

Kilgariff starts laughing. “The best was one lady who told us we could quit any time… She meant it in a lovely way. She just grabbed both of us and said: ‘You don’t have to do this, you can stop at any time.’” Given that it’s been two years from podcast to bestselling book, the whole thing must feel like being on a rollercoaster. Hardstark laughs: “We know it can’t last, so it’s all about having as much fun as we can while it does. It’s been a life-changing experience, but we know how fortunate we are so we’re just enjoying the ride and getting high off it.”

“I think we should just keep doing it until we burn out completely,” Kilgariff says with perfect deadpan delivery. “Although, really, we have no choice but to do a musical. We don’t want to and it’s not a good idea, but we’re almost certainly going to have to. It is what it is.”

Stay Sexy & Don’t Get Murdered by Karen Kilgariff and Georgia Hardstark (Orion Books, £14.99) is available from guardianbookshop.com for £13.19