Last August saw the Girls Aloud singer reveal that she had been diagnosed with breast cancer, which had spread to other parts of her body.
The 39-year-old went on to inform fans in December that she had started writing an autobiography while going through treatment.
Harding shared to Twitter on Monday: "Hi everyone! I hope you are keeping safe and as well as possible with everything that’s going on.
"I know I’m not really that present on here which I promise I’ll try to get a bit better at, as honestly it means the world to me when I come on and see all your well wishes."
Watch: Sarah Harding diagnosed with advanced cancer
She continued in a thread: "Thank you for the love and support, on bad days it helps me so much. So here’s a little update from me… Mum, the dogs and I had a really lovely but quiet Christmas together, which was different to my usual, but seemed a fitting way to end such a strange year."
Harding then stated that she had managed to finish the book, which is titled Hear Me Out.
She explained it was named after the song she wrote on the second Girls Aloud album What Will the Neighbours Say? as the lyrics "have always meant a lot" to her.
Harding added the book was set for release on 18 March.
Her former Girls Aloud co-star Kimberley Walsh replied to her post, saying: “Proud of you and can’t wait to read it.”
She received messages of support from Walsh and the other members of the group that rose to fame on ITV’s Popstars: The Rivals with Cheryl, Nadine Coyle and Nicola Roberts reaching out back when she disclosed her diagnosis.
With additional reporting by PA.