Helen Bailey had described Ian Stewart as her "happy ending".
Her husband, John Sinfield, had drowned while the couple were on holiday in Barbados in February 2011 and Ms Bailey met Stewart through a Facebook bereavement group later that year.
She wrote that "entirely innocent support and friendship turned into romance".
The author chronicled her life as a widow through a blog, Planet Grief, which became a book, When Bad Things Happen in Good Bikinis, in 2015. She nicknamed Stewart the Gorgeous Grey-Haired Widower (GGHW) and among the dedications was a message to him which read: "I love you. You are my happy ending."
Ms Bailey had not been looking for new love after her husband's death and wrote that she was shocked when she had a "disgustingly inappropriate thought" about Stewart while walking through the underwear department of M&S.
Feeling "sleazy, guilty and ashamed" she encouraged him to date other women but their daily messages continued.
Ms Bailey later wrote that she would have been "devastated" if he had met someone else and they eventually arranged a date - lunch at the National Portrait Gallery and an afternoon screening of Tinker Tailor Soldier Spy in Leicester Square.
It was a "tearful, angst-ridden disaster, and an acknowledgement that it was all too soon", she wrote.
Their friendship was put on hold. "There were no more funny emails, no witty texts. Life felt even darker than it was already. I missed him," she wrote.
After a break, they started "uncomplicated" dog walking again.
During one walk on Hampstead Heath, near Ms Bailey's London home, Stewart got stuck in mud and the author said she was doubled up with laughter. He laughed, too, and she recalled thinking: "I don't want to lose that man from my life."
She said he "was wonderful" - helping with the plumbing and electrics and listening as she "wailed" - after he drove her to Broadstairs, Kent, to check on her holiday cottage in late 2011.
It was her first visit since her husband's death but she wrote that Stewart understood as his wife had also died suddenly.
Ms Bailey even braved her fear of taking another beach holiday when they went to Portugal and had a "wonderful" time.
She wrote that she admired Stewart who was "a man who can see an upside in the most dire of situations" and who reassured her they would have "wonderful memories" if the worst happened.
They were both "vulnerable, emotional wrecks" when they met and, unsure if the relationship would survive, Ms Bailey was reluctant to make it public but finally revealed it to her readers in March 2012.
The following year Ms Bailey and her miniature dachshund, Boris, made a "fresh start" in a new home in Royston, Herts, with Stewart and his two sons.
The large old house "felt like home", she wrote, although it was not always easy "living in a home that came together through sudden death".
Ms Bailey said one of Stewart's sons had suggested she end the book by writing "And they all lived happily ever after".
Instead she signed off by reassuring grieving readers "It will all be OK in the end".
Three years after she moved to Royston, Ms Bailey was found dead.
:: When Bad Things Happen in Good Bikinis by Helen Bailey is published by Blink Publishing.