Letters written by former South African president Nelson Mandela during his 27 years in jail reveal the agony of being separated from his family and how he never wanted to become South Africa's president.

In one excerpt from the book, "Conversations with Myself", released on Tuesday, it is clear that Mandela had become wary of his growing international reputation while being held in jail on Robben Island.

"One issue that deeply worried me in prison was the false image I unwittingly projected to the outside world; of being regarded as a saint.

"I never was one, even on the basis of the earthly definition of a saint as a sinner who keeps trying," he wrote.

The book, compiled by the Nelson Mandela Foundation from personal letters, interviews and an unpublished sequel to his autobiography, contains a foreword by U.S. President Barack Obama.

In the book, Mandela also says that that he only accepted becoming president of South Africa after senior leaders of the African National Congress put pressure on him.

"My installation as the first democratically elected President of the Republic of South Africa was imposed on me much against my own advice," Mandela said.

Through Mandela’s peaceful takeover of office and becoming the country’s first black president, he was able to begin the painful reconciliation between blacks and whites in the country, making it the cornerstone of his presidency.

However, in the book, an important theme is his concern about the effects of his imprisonment on his family.

Mandela shows his anguish and frustration in one letter to his former wife Winnie Madikizela-Mandela, who was banished to a remote town and harassed by apartheid security police.

"I feel I have been soaked in gall, every part of me, my flesh, bloodstream, bone and soul, so bitter am I to be completely powerless to help you in the rough and fierce ordeals you are going through".

Mandela also writes about not being able to attend the funerals of his mother and of his son, who died in a car accident in 1969.

"Though I had never hoped to succeed, my heart bled when I finally realised that I could not be present at the graveside -- the one moment in life a parent would never like to miss".

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