In March 2015, Elizabeth Uviebinené had a brainwave that a less determined 22-year-old might have dismissed as a water-cooler pipe dream. It was ignited by a single chapter in a book by Sheryl Sandberg . “I’d always devoured self-help books growing up – books like Lean In,” says Uviebinené. “These were written by white women and were great but they didn’t have the added complexities of how to be a black woman and get ahead. It was like we didn’t exist in these books. Sandberg had one chapter in her follow-up book [Option B] about a black woman’s experience and it sparked something in me. A need for a sisterhood. I wanted to bottle it.”

The bottling, she thought, would come in the form of a book – a bible no less – to offer black teenagers and women the kind of advice she would have liked to have received growing up, to help navigate her way to a bigger, freer life.

The problem was that she was a marketing manager and not a writer. So the perfect person to write it, she decided, was her best friend, Yomi Adegoke, then also 22 and working at Channel 4. “Because Yomi’s a journalist, I thought she would write it and I would market it,” says Uviebinené. “But she sold the idea back to me to both write it together.”

So that’s what they did. It did not matter that they were both working long hours in highly competitive careers – they wrote every evening and weekend for the next year. Nor that neither had written a book before – they simply Googled “how to write a book” and set up focus groups with friends to discover the breadth of what they needed to research.

Taking the dictum to work twice as hard to get half as far as their white counterparts – which they believe is one of the only ways for black communities to succeed in Britain – they secured a book deal a year later, despite the outrageous odds against two black women with no inside connections to an overwhelmingly white, elitist publishing industry.

It was not just any book deal either. Slay in Your Lane: The Black Girl Bible is arguably the book for 2018, fought for in a nine-way publishers’ auction with Uviebinené and Adegoke receiving – to their astonishment – a five-figure sum.

Uviebinené was at work when she got the phone call from her agent. She hid in the office loos and made a breathless call to Adegoke. “I was trying to Whatsapp Yomi from the top of Canary Wharf tower, trying to explain what had just happened and I felt so overwhelmed.”

Adegoke was celebrating her 25th birthday in Rio de Janeiro. “I had just broken up with a boyfriend and I wanted to go away after one year of working intensely. I was having a really great time and I’d lost my phone for the previous few days.”

Both are 26 now. They met as 18-year-old freshers at the University of Warwick. There they found themselves marooned and misunderstood in an almost uniformly white environment that seemed unconsciously hostile. Adegoke took a year out from Warwick after suffering from depression (she turned this into a productive experience by founding a magazine titled Birthday).

“Her confidence was mistaken for arrogance,” explains Uviebinené. “It was ‘us’ against everyone else.”

“Yeah,” adds Adegoke, “We’ve both been through a lot. It’s one of the reasons we get on so well. There is a lot of respect there.”

Both are of Nigerian heritage – Uviebinené’s father came to Britain when she was two (leaving behind her mother, from whom she is estranged) and she is the eldest of six children. Adegoke was born and raised in Croydon, south London, and has two sisters to whom she is very close. Uviebinené has become a kind of third sister: she moved in with Adegoke’s family in 2016 and has lived there since.

The book looks at the mechanics of succeeding at school and in the workplace within a greater system that is structurally tilted against British black communities (they back this up with a barrage of statistical research). In it, they talk of the “concrete ceiling”, “impenetrable glasshouses”, the “long, back-door route into success” and of such flagrant bias at work that some black candidates have fared better by sending in job applications using aliases.

They grapple with issues around health, dating and representation that are specific to black female experience. What is most impressive is the book’s penetrating research as well as its argument. “Well, if you are going to call a book ‘the black girl bible’, you have to do your research, and incorporate the full cross-section of black women’s concerns,” says Uviebinené.

Adegoke nods vigorously. “In the past year and a half, our lives have been dedicated to making sure we get this right. It’s been the most challenging thing we’ve ever done. I don’t know that I would have a book deal without Elizabeth.”

“We are each other’s champions,” Uviebinené shoots back. “Yomi is one of the most confident and articulate people I know.”

There are many such tender exchanges. They are friends who seem more like sisters, though in temperament they are yin and yang. Uviebinené turns up bang on time and is calmly composed and earnest. Adegoke, who arrives late, is a whirlwind of energy.

They do not talk over each other or finish each other’s sentences but seem to know what the other will say. Much of this sisterliness and fierce spirit of “strength in adversity” is channelled into their book. Uviebinené says it was written for her younger self, and all those girls and women who, like her in those formative years, are trying to figure out who they are in the world, and how they can succeed at life. “When I was 16, I’d write notes to myself every time things got hard on the type of person I wanted to be. I was always looking to what life held for me. So this book is about what I wanted to know when I was 16, and 18, and 21, that popular culture wasn’t telling me.”

It also gives young black girls a host of role models or “cheerleaders” that the authors believe are badly needed. They interview 39 accomplished black women for their perspectives on success, from Malorie Blackman to Laura Mvula, June Sarpong, Denise Lewis, Jamelia, Margaret Busby and Estelle. Young girls need to “see it to be it”, they say, quoting Dr Karen Blackett, chair of MediaCom, from the book’s foreword, and such cheerleaders were not always visible while they were growing up.

We didn’t think, ‘This will work because black girls have traction right now' Yomi Adegoke

Uviebinené found inspiration in older women who accomplished great things, while for Adegoke, it came from closer to home. “My older sister, Yemisi, is a journalist for BBC Africa. She always told us dark skin was beautiful. She made sure we didn’t have issues around it. She was like a third parent, making sure we grew up confident.”

Since Uviebinené’s lightbulb moment in 2015, the subject of diversity has exploded in publishing. Bestselling books by Reni Eddo-Lodge and Afua Hirsch have shown the industry that there is an immense appetite for a conversation around race and inequality in Britain.

What do they feel about this moment? “Liz was trying to set the agenda, not follow it,” says Adegoke. “So we didn’t think: ‘This will work because black girls have traction right now.’”

But they welcome the drive for better representation. “We see it as a blessing that our book has coincided with a trend for diversity,” says Adegoke. “We can see that just as it is in fashion now it can fall out of fashion. But then again, black women are not going anywhere. We don’t fall off the planet when we’re not in fashion. We’ll still here and we believe we’ve written a good book, and that people will read it, even if people get bored of diversity.”

Is it relevant to non-black readers, too, and inspiring girls across the range? “I think non-black people should read it to understand the experience of what it means to be a black woman. I have been reading books that have not been written for me all my life,” says Adegoke.

For Uviebinené, intersectionality is key in feminism: “You can sing from one voice but it’s also not as straightforward as that. You need to see your privilege for what it is first. Feminism for me is about gaining choices and breaking down the insidious structures that hold us back. White women tend to want to be more like white men in their privilege. I don’t want to be more like white women.”

Do they think unequal structures are slowly being broken down, and that Britain is changing? After all, a mixed-race woman recently married into royalty. “I don’t think Meghan Markle massively marked a change. It’s Harry’s choice, not an institutional choice, though I do think she could be an agent of change for women,” says Adegoke. “But if Harry had brought home a girl who looked like us – black girls with kinky hair and big noses – it would be a very different conversation.”

They swap a look again, a kind of signal that they have each other’s backs. What about boyfriends, I ask. Where do they sit within their friendship? “My boyfriend is basically a version of Liz – a short, dark-skinned Taurus.” Does he get on with Uviebinené? Adegoke shrugs as if this is a no-brainer. “They have to love her. Of course.”

• Elizabeth Uviebinené and Yomi Adegoke will join Afua Hirsch for a Guardian Live event on Wednesday 25 July at Kings Place, London N1

• Slay in Your Lane: The Black Girl Bible is published next month by Fourth Estate (£16.99). To buy a copy for £14.44 go to guardianbookshop.com or call 03303 333 6848. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99