Author Bret Witter reads from and signs copies of the book he co-wrote with Susan Spencer-Wendel, “Until I Say Good-Bye: My Year of Living with Joy.” The book is a moving and inspirational memoir by a woman who makes the most of her final days after discovering she has amyotrophic lateral sclerosis.

7 p.m. Tuesday, April 23. Jimmy Carter Presidential Library & Museum. Free. 404/865-7100. 441 Freedom Parkway, Atlanta. http://www.jimmycarterlibrary.gov/events.

Susan Spencer-Wendel’s right thumb — the finger she depended on to type her memoir last summer — doesn’t work very well anymore, but it has one more important job to do: Help her sign copies “Until I Say Good-bye,” her 364-page life story.

Spencer-Wendel’s muscles are withering from amyotrophic lateral sclerosis, or Lou Gehrig’s disease. It has robbed her of the ability to slather on lipstick and dance in 4-inch stilettos and hug her three children tight, but not her ability to laugh at herself or enjoy life.

The book, which was released last month and co-written by Decatur resident Bret Witter, has kept her busy at book signing events. It had climbed this year to as high as number 3 on the best seller list.

Before he met Spencer-Wendel, Witter worried whether they could meet the deadline. The disease has already robbed her of most of her ability to move. They had only months to write the book, for which she was paid more than $2 million.

She’d tap out three or four pages a day on her iPhone, then ship them to Witter.

He said, “I thought, ‘This is going to be impossible. We only had four months. How can she possibly do this?’”

“You worry too much,” she would reply.

Witter said, “She could only see 16 words at a time on her phone, so my job was to give her context — to move the furniture around until it was all in the right place,” he says.

In three months, she wrote 89,000 words with the one finger that could still type.

The result is an emotional roller-coaster of a memoir — not a self-help book and not a dreary book about a disease. It is a story about relationships, Witter says, a “love story about a woman who is in almost every way an ordinary person, and everyone can relate to her.”

After her diagnosis, she went to see the Northern Lights with her lifelong best friend, Nancy Maass Kinnally, and took a 20th-anniversary trip to Budapest with her husband, John. She went on a cruise with her sister, and took trips with her three children.

One of the most poignant chapters describes her visit with daughter Marina to the famed New York bridal store, Kleinfeld’s, site of the TV show “Say Yes to the Dress.” She thought Marina might want to try on a few dresses.

“I simply wanted to make a memory. I wanted to see my beautiful daughter on her wedding day. I wanted to glimpse the woman she will be,” Spencer-Wendel writes. “Maybe I would cry. Mothers cry, right? But I knew I would laugh, too. Because I would be with Marina. I would be imagining her happy.”

And this is what amazed Witter about Spencer-Wendel, over and over again, as she wrote about her adventures and her relationships, both strong and delicate: “She never set an expectation in her mind that an experience has to be a certain way or it’s a failure. She said: ‘I’m going to enjoy the experience, whatever happens.’

“She wrote the book the same way,” Witter said, “She let the experience of writing it guide her. When you live like that, without expectations, you enjoy life more.”

That’s the Buddhist way, and Susan’s way, too — the only way the 46-year-old can “get my Zen on.”

“To want something you can never have,” Susan writes, “that is the path to the loony bin.”

“The book’s message,” Spencer-Wendel says, “is to live with joy. Set your intention and do it. And accept nature. No matter what happens to you…accept nature.”

As her body weakens, her sense of peacefulness becomes stronger. This is obvious to readers of the book — who see her transition from a frenzied working mom to a woman who sets her mind moment-by-moment to seek peace.

Witter says his work with Spencer-Wendel changed him.

“She is part of me now,” he says. “Her biggest lesson — it feels like a cliche, but it’s most important — is don’t make excuses and say you can’t do something. Do what you want to do and enjoy what you have. If Susan can enjoy her life, why am I putting expectations on my life and worrying if I meet them or not? Why am I not enjoying my life?”