Bernard Madoff's decades-long fraud might not have been his only secret. A new book says he had a two-decade affair with one of his investors.

The memoir, Madoff's Other Secret: Love, Money, Bernie, and Me, was written by Sheryl Weinstein, whose relationship with Madoff spanned more than 20 years while both were married, said John Murphy, a spokesman for publisher St Martin's Press. It goes on sale on 25 August.

Madoff (71) is serving 150 years in prison for defrauding investors.

Weinstein said she met him at a business meeting when she was chief financial officer for the charitable women's organisation Hadassah, where she had a role in investment decisions.

A lawyer for Madoff's wife, Ruth Madoff, said his client did not know about the "alleged affair".

Weinstein said she viewed meeting Madoff 21 years ago "as perhaps the unluckiest day of my life". She said her losses had forced her to sell her Manhattan home and devastated her, her husband of 37 years, her son, her parents, her in-laws and everyone who depended on them.

She called Madoff "that terror, that monster, that horror, that beast... an equal-opportunity destroyer".

In her correspondence with the court, Weinstein made no mention of the affair, though she did write in a request to speak at sentencing that she wanted to address Madoff because "I think the personal connection may be more difficult for him to ignore".

Her husband, Ron Weinstein, said in a June letter to the court that all the money the couple had saved was lost by Madoff and that their marriage was strained.

"My wife has been a basket case," he said. "And we are both very depressed."