The best friend and business partner of Kate Spade has broken a seven-year silence about the fashion icon's shock suicide, delivering an emotional account of her final hours.

In a memoir out next week, Elyce Arons rejects controversial allegations that Spade was driven to take her own life by the collapse of her marriage or that the designer refused treatment for chronic mental illness.

Dismissing 'surface-y, ready-made explanations' - including claims made by Spade's older sister - Arons insists her friend's death should be seen as an 'incomprehensible' mystery.

Spade had made a personal promise she would never take her own life, reveals Arons, who writes that: 'She had said definitively to me, 'I would never, ever do that'.

Kate Spade's simple, almost child-like designs – at first displayed on handbags, but later on clothes, perfume and accessories - were popularized by Hollywood stars and even royalty, with clients including Catherine, Princess of Wales, and her younger sister, Pippa.

Yet neither Spade or Arons, co-founder of the brand, came from privileged or well-connected backgrounds.

Arons's memoir, 'We Might Just Make It After All' charts a friendship which began at college in the mid-West, produced runaway, multi-million-dollar success, but ended with a phone call saying a housekeeper had found 55-year-old Spade unresponsive in the bedroom of her New York apartment.

Far from being on the brink of suicide, Spade appeared happy in her final hours, writes Arons, who recalls chatting on the phone just one day earlier, on June 4.

Elyce Arons (L), the best friend and business partner of Kate Spade(R), has broken a seven-year silence about the fashion icon's shock suicide, delivering an emotional account of her final hours. 

Kate Spade's simple, almost child-like designs – at first displayed on handbags, but later on clothes, perfume and accessories - were popularized by Hollywood stars and even royalty. (Pictured: Kate Spade in 2004). 

Spade had been looking forward to a vacation in Napa with her husband and their 13-year-old daughter.

'They were working out their marriage issues,' reflects Arons. 'I can tell you that Katy and Andy loved each other to the ends of the earth. Katy worshipped her daughter. She'd dealt with professional and personal wobbles throughout her life.'

Then, in the middle of a normal, upbeat conversation, Spade hung up to take another call.

'She told me she would call me back. It was just inconceivable to me that we would never speak again.'

When she heard the news about her friend's death the following morning, Arons 'let out a cry of distress from a grief so deep that I barely remember what happened next.'

Spade's older sister, Reta Saffo, has dominated coverage of the tragedy, claiming that Spade refused hospital treatment after being diagnosed with bi-polar disorder for fear it would hurt her 'happy-go-lucky' image.

According to Saffo, her sister was 'self-medicating with alcohol' and had become obsessed with television coverage of Robin Williams's 2014 suicide.

'She kept watching it and watching it over and over. I think the plan was already in motion even as far back as then,' Saffo has said.

But Arons suggests that, far from having a death wish, Spade had been fighting to survive: 'I talked often with Katy about her struggle with depression...

'She was actively seeking help with specialists, and we understood the goal was to mitigate the times [she] was carrying that deep sadness she couldn't seem to shake, and which had weighed heavily on her in recent years.

'It has taken me more than seven years to write about my best friend, Katy, since her passing. I knew it would be important to do so before too many details slipped away.

'Why would someone with everything - family, friends, talent, fame - take her own life?

'I can tell you those of us closest to Katy knew she was battling depression in her later years, but none of us ever envisioned her taking that tragic step.'

The two women first met at college in Kansas and bonded in unlikely circumstances.

Despite Spade's later reputation for simple elegance, Arons recalls the two young women were caught short after drinking beer at a frat party and relieved themselves behind a bush on campus - only to be stung with poison ivy in the process.

They spent the next two weeks applying calamine lotion to each other's backsides.

After college Spade and Arons moved to New York and found work in fashion journalism and photography. Then, in 1993, they were inspired to found the Kate Spade fashion business - and achieve near overnight success.

Kate Spade (R) had been looking forward to a vacation in Napa with her husband Andy Spade (L) and their 13-year-old daughter at the time of her suicide.

After college Spade (middle) and Arons (R) moved to New York and found work in fashion journalism and photography. Then, in 1993, they were inspired to found the Kate Spade fashion business - and achieve near overnight success.

Their $250 bags became so popular that fake versions proliferated. The memoir recalls how, one day, Spade saw counterfeits for sale on blankets outside Barneys, the high-end Manhattan store.

'Katy and Andy happened to be walking by...and someone was selling knockoffs right outside on Madison Avenue,' writes Arons.

'A woman and her ten-year-old daughter were looking at all the fake Kate Spades, and Katy said, 'Those aren't real.'

When asked how she knew, Spade replied: 'Well, I'm Kate Spade. These are fake bags. You can go into this store and buy a real one right now.'

The woman was not impressed, however, telling Spade that the fake ones 'look the same'.

The designer was enraged, but her husband Andy told her imitation was the most sincere form of flattery.

Fakes or no fakes, the brand became hugely profitable and it wasn't long before Nieman Marcus snapped it up. The department store chain bought 56 per cent stake in 1999 for $33.6million and the remaining 44 per cent in 2006 for $59 million.

Spade herself was estimated to have earned some $46m from the business.

By the turn of the century, however, the pressure was starting to tell. Because the brand bore her name, Spade was constantly on the road with promotional tours. Tensions between the two women grew with Kate complaining she felt lonely and 'abandoned' by her friend.

'Her fame was another factor in our drift,' continues Arons.

'At this point Katy had profiles and interviews written about her in scores of national magazines. She and Andy had dozens of invitations weekly to every conceivable event.'

Nieman Marcus bought 56 per cent stake in Kate Spade in 1999 for $33.6million and the remaining 44 per cent in 2006 for $59 million. Spade herself (pictured with her handbag in 1998) was estimated to have earned some $46m from the business.

Because the brand bore her name, Spade was constantly on the road with promotional tours. Tensions between the two women grew with Kate (pictured in 2001) complaining she felt lonely and 'abandoned' by Arons.

She recalls how, on one memorable occasion, the two best friends and their husbands arranged to meet at Pastis, a restaurant in the Meatpacking District of Manhattan. But Kate and Andy Spade turned up two hours late - with a group of strangers in tow.

'They would never have done anything to hurt our feelings,' she continues. 'But that experience was emblematic of the change in our relationship.'

Spade stepped back from the business, intending to focus on motherhood. But in 2016, with Arons, she launched a new collection of luxury footwear and handbags under the brand name Frances Valentine.

The book recalls on one notable disaster with a shipment of shoes so narrow, American customers couldn't squeeze into them.

'In America, the typical woman's foot is a "B" width. But our new Italian factory manufactured them all in a narrow width', says Arons.

'It cost a fortune in returns.'

By 2017, Frances Valentine sales were strong and it was starting to look as though 'lightning might just strike twice'. But one year later, Katy had gone.

'Losing my best friend for life - the woman who shared my sense of humor, who'd been my constant companion at school, at work, at dinners, on the phone, in my house, on vacations - was like losing your face in a mirror,' concludes Arons.

'It was disconcerting, disturbing and very lonely. I plagued myself, asking 'Why?'

'It pains me deeply to imagine Katy sinking into a deeper depression and not talking about it with anyone. She was private about many things, even with me.'

We all have dark moments and periods, says the memoir, and in one fatal moment, Kate Spade lost hope.

Spade stepped back from the business, intending to focus on motherhood. But in 2016, with Arons, she launched a new collection of luxury footwear and handbags under the brand name Frances Valentine. (Pictured: Kate Spade with her daughter Beatrix Spade and friends in 2013).

'I can tell you that Katy and Andy loved each other to the ends of the earth. Katy worshipped her daughter,' Arons revealed. (Pictured: Andy and Kate in 2006). 

Arons still was in disbelief when, four days later, she went to her friend's apartment to collect some accessories.

'As I put my hand on the closet door handle, I couldn't help but think Katy was going to jump out of the closet and scare the sh*t out of me.

'The hairs at the back of my neck stood up as I held my breath and opened the closet door. Hundreds of moths flew out in a black wave, coming right at me. I shrieked, full-lung power, and stumbled backward....

'Any second, [Kate] was going to come up behind me, tap my shoulder, and say, 'Gotcha!'

But Kate Spade, of course, was 'really and truly gone.'

'Katy left us with many questions,' concludes her friend.

'All of us who have loved her have had to find a way to make peace with her incomprehensible choice.'