The royal speaks about falling in love with Bonas, as well as their "painfully awkward" first kiss and how their break-up left him feeling "miserable".
Here is everything you need to know about their relationship.
Who is Cressida Bonas?
Cressida Bonas is a 33-year-old model and actress.
The only child of entrepreneur Jeffrey Bonas and socialite Lady Mary Gaye Curzon, she was born in Winchester, Hampshire, and studied dance at the University of Leeds between 2008 and 2011.
Bonas has modelled for fashion brands including Burberry and Mulberry, and appeared in films such as Tulip Fever and The Bye Bye Man.
Her most recent acting credit was in the ITV true-crime drama White House Farm, which was broadcast in 2020.
Despite her surname, she has no ties to the fashion and homeware brand Oliver Bonas.
How did Harry meet Cressida Bonas?
Harry was introduced to Bonas by his cousin, Princess Eugenie ('Euge' as he calls her), at a musical festival in the summer of 2012.
In Spare, Harry writes that he was "still suffering a broken heart" following his split from long-term girlfriend Chelsy Davy, but that there were "sparks" between him and Bonas.
He adds: "Soon after that day we went to dinner. She told me about her life, her family, her dreams. She wanted to be an actress."
He remembers her being so "soft-spoken and shy" that "acting was the last profession I'd have imagined for her".
Weeks later, they shared a "painfully awkward" first kiss after their second date. "I leaned in to give her a kiss, but my aim was off," he writes. "I could take out a cactus from three miles away with a Hellfire missile but I couldn't quite find her lips."
What does he say about their relationship?
Despite the awkward kiss, Harry and Bonas went on further dinner dates and quickly became an item.
"Before I knew what was happening, we were a thing," he writes. However, due to Harry's service in the Army, the couple could only see each other at weekends.
Shortly before his deployment in Afghanistan in late 2012, Harry was photographed naked while playing 'strip billiards' with friends in Las Vegas.
Reflecting on the scandal in Spare, he says he apologised to Bonas for "embarrassing her" and gave her an "careful and abashed explanation" of what had happened.
When he returned from the war zone and reunited with Bonas, Harry says there was some "some distance" and "some stiffness" between them.
The couple attended their first official engagement together in March 2014, but broke up following a ski trip to Kazakhstan later that month.
Harry says he felt "miserable" when he "realised that we weren't a match". He adds: "There was massive affection, deep and abiding loyalty – but not love everlasting."
In the book, Harry also reveals he considered marrying Bonas, and that his brother William and sister-in-law Kate had "mentioned, pointedly, repeatedly, how much they liked Cressida".
He also writes that he cried while opening up to Bonas about his late mother, Diana, which "accelerated" the bond between them.
She was his last serious girlfriend before he met Meghan in 2016.
Where is Cressida Bonas now?
Cressida Bonas is thought to remain on good terms with Harry. She attended his wedding to Meghan in 2018, as well as Princess Eugenie's wedding in the same year.
In an interview in 2020, she said she "still has to contend with" people wanting to talk to her about Harry rather than her work.
She told ES Magazine: "No-one likes to be labelled, no matter what it is about or where they come from.
"The hurdles and barriers for me are when I'm trying to do my work and people want to talk about him.
"I work very hard and love what I do – I just want to continue. But it is still something I have to contend with. It is what it is. People will perhaps always ask me about it."
Later that year, she married property investor Harry Wentworth-Stanley, and in November 2022 she gave birth to her first child, a boy.
She wrote an article for the Sunday Times revealing her journey with IVF after facing difficulties in conceiving, saying she felt "as if my body was failing me".
Watch: Cressida Bonas arrives for Princess Eugenie's wedding