A mother of three dying of motor neurone disease has written an 89,000 word book on her iPhone, using the only part of her body below the neck that she can still move – her right thumb.

For more than 20 years, Susan Spencer-Wendel's job was about telling other people's stories, recounting tales of tragedy and perseverance and winning awards with her writings about justice and injustice.

Juggling a career as a newspaper reporter with the demands that go with being a married mother of three, she felt lucky and happy, never considering that her expectations of watching her children grow up, graduate and wed, or her dreams of grandchildren and retirement, would be anything but plain sailing.

But in the summer of 2009, her ordinary life turned extraordinary when she suddenly noticed that her left hand was withered. The subsequent diagnosis of Amyotrophic Lateral Sclerosis, or motor neurone disease, delivered in 2011 after the symptoms had begun to creep through the rest of her body, brought with it a death sentence.

It left her with one more story to write. Because while the neurological disease would go on to paralyse her muscles, robbing her of her mobility and reducing her speech to a heavy slur, the year or so she had left would be about "living with joy, and dying with joy and laughing a helluva lot in the process", she resolved.

Her refusal to face death with fear was made all the more remarkable by the manner in which she committed her memoirs to the record. Determined not to be beaten by her crippled body, but to leave behind an inspirational message about the things in life that matter most, and the art of letting go, Mrs Spencer-Wendel, 46, patiently tapped out all 89,000 words on her iPhone, using the only part of her body below the neck that she can still move – her right thumb.

"You want to keep doing the things you can do for as long as possible. You realise, 'Hey, I can't jump, I can't do up my own shorts or pants, I can't walk, I can't do this or that.' You don't want to give up until you absolutely have to," she said, sitting beneath the shade of a thatched canopy in the garden of her home in West Palm Beach, Florida.

Her book, Until I Say Good-Bye, to be published on both sides of the Atlantic this week, was purchased by HarperCollins in the US for a sum in the region of $2 million. It will be translated into more than 20 languages, distributed on four continents and is forecast to become a bestseller. Universal Pictures have bought the movie rights for a further $2 million.

"People have been sharing their stories with me forever," she says. "Now I'm sharing mine. I want it to be a force for good."

The book chronicles what she calls her "final, wonderful year". She journeyed to the sub-Arctic in search of the Aurora Borealis, viewed America's last space shuttle launch, and reunited with her birth mother in California more than four decades after she was given up for adoption.

She tracked down relatives in Cyprus of the late biological father she'd never known, swam with dolphins and took her teenage daughter to try on wedding dresses at New York's finest bridal shop, knowing that she will not live to see her choose a husband and marry for real.

"'Bucket List' is too silly a term. My philosophy is 'Do what you delight in – and do it no matter what' – so I did," she told The Sunday Telegraph in an exclusive pre-publication interview. "You can't fight nature. Don't fret about what you can't control - it can't be undone. Death will be a blessing, I believe that every day, but I haven't let that get in my way. There's death, but first there's life."

Like most of her muscles, she is fast losing control of her tongue and throat, making speech difficult – both for her to deliver and for others to understand – and causing her periodically to start choking. She is reliant on her husband, John, 48, to move her around, lowering her into her wheelchair or standing her out of it, to dress her, wash and groom her. She can no longer even lift her arms to hug her children, Marina, 15, Aubrey, 11 and Wesley, nine, for whom she intends the book as a celebration of her life and the memories they made together.

ALS causes nerve cells to waste away and die, meaning that messages are no longer carried from the brain to the body's muscles. By June last year, Mrs Spencer-Wendel's hands were so deformed and near-useless that it was too much for her to even move them back and forth across her iPad keyboard.

Knowing that she only had a short window of opportunity left before she lost all movement completely, she embarked on her book project using the "Notes" section of her mobile telephone, clutching it in her twisted and gnarled left hand. "I would type each letter with my right thumb – tap! tap! – the only digit I could control," she explains in the book.

When people came to visit, she would ask them to read aloud what she had crafted on the screen. "I could not hug them. I did not go out for meals with them or to the beach. I could not walk the yard or have a conversation for more than a few minutes… These were my conversations," she notes.

"I was speaking to my family and friends with my written words and I was reliving the moments. Meeting John. My children's births. The peace I embraced inside… Writing this book was not work. Like each journey I took during the year, it brought me joy. It kept me alive."

When it was complete, she was thrilled by the accomplishment – "I felt as if I'd pulled myself up a mountain with nine fingers tied behind my back."

Sitting in her patio chair, her dog Gracie napping at her feet and the breeze ruffling the tropical vegetation around her, she is a picture of serenity. But she admits to the occasional low moment, and to tears when the manuscript was complete.

"It was panic, it was 'What do I do now?'" she explained. "The book was a blessing. It was a counter-balance to how bad the diagnosis was, a balancing out of fortunes, good and bad."

Her husband, John, added: "A book is what happens when you take a woman who can't sit still and make her sit still. ALS hasn't done anything to her mind."

The book, they both say, was a "terrible trade-off" – Mrs Spencer-Wendel is losing her life but leaving behind financial stability for her husband and children, "the best possible outcome of the worst possible scenario" as she calls it.

Yet they have also rejoiced in the realisation that the book will bring greater attention to ALS, also known as Lou Gehrig's Disease after an American baseball star whose emotional withdrawal from the sport in 1939 pitched the illness into the public conscience.

"I consider myself the luckiest man on the face of the Earth. I might have been given a bad break, but I've also got a lot to live for," Gehrig told the crowd at his final match, after ALS ended his 17-year career.

Mrs Spencer-Wendel took his lesson and paid it forward. "We don't go around as a family, all 'Woe is me.' I don't fret about the small things. I have to let things not drive me nuts, otherwise everything would drive me nuts. The message is 'Quit complaining. Accept. Live with joy."

Until I Say Good-Bye is published by Two Roads on March 14, £16.99, also in ebook