Cowell announced his split from the make-up artist, whom he met on the set of American Idol, in February after two years.
Hussainy is said to be upset about Cowell's remarks about her in journalist Tom Bower's book Sweet Revenge: The intimate Life of Simon Cowell, in which he described their engagement as a "big mistake".
"Mezhgan was very hurt by some comments in the biography, particularly a quote from Simon's mum Julie that said Mezhgan made her stomach churn," a source is quoted as telling The Mirror.
"She was embarrassed, feeling she'd been made to look a fool, and she plans to put her side across, which is likely to leave Simon squirming."
Photo gallery - Simon Cowell's ex-girlfriends:
Copyright: Rex FeaturesThe memoir will reportedly describe Hussainy's early life in Afghanistan and career as a make-up artist in Hollywood, as well as revealing the argument that led to the end of the couple's romance.
The insider added that Hussainy is "not shy about speaking her mind" and "doesn't allow herself to be walked over".
However, Hussainy's representative denied the claims, insisting that the 39-year-old currently has no plans to write a book.