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Margaret Trudeau is writing a book on her long-time battle with bipolar disorder. ((Graham Hughes/Canadian Press))

Margaret Trudeau is writing her third autobiographical book, this one about her struggle with bipolar disorder.

It will be available in bookstores in the fall of 2010, HarperCollins announced Wednesday.

In 2006, the ex-wife of the late prime minister Pierre Elliott Trudeau announced that she had been suffering from bipolar disorder for many years. She has since become an outspoken mental health advocate on the lecture and interview circuit.

Bipolar disorder leaves sufferers with extreme mood swings, from euphoric highs to rock-bottom lows of depression.

Trudeau, 60, has said the disorder played a pivotal role in the breakdown of her two marriages – to her first husband, Pierre Trudeau, and to Ottawa real estate developer Fried Kemper.

A spokeswoman for HarperCollins said Trudeau's forthcoming book will "remain an important part of the Canadian consciousness for years to come."

Trudeau's first book, Beyond Reason, published in 1979, delved into her eight-year marriage to Pierre Trudeau. The couple married in 1971 when she was 22 and he was 51 years old.

Her second book, Consequences, published in 1982, describes her life following the end her first marriage.

Trudeau now lives in Montreal.