The couple, who met at art school, were together for 10 years, right through the peak of The Beatles' global stardom.
Although Cynthia did write a book about their marriage in the 1970s, she claimed she was too afraid of John's power to delve too deeply into their time together.
In John, she has told of everything from Lennon's confession that he slept with other women throughout their marriage, to how his family struggled to fight for their share of his estate when he died.
She also reveals the day she walked in to find her husband sat with Yoko Ono, and realised they were having an affair and how the two women have battled ever since.
Cynthia, who went on to marry three more times, now lives in Spain with her husband Noel where she writes and paints.
She added: "Writing the book was quite heavy duty. But I had to write, I felt I had to get it all out and it was my time to tell my story.
