The book "Why He's a Saint" also includes previously unpublished speeches and documents written by John Paul, including one 1989 signed memo in which he said he would resign if he became incapacitated.
The book also reported for the first time that John Paul forgave his would-be assassin in the ambulance on the way to the hospital moments after he was shot on May 13, 1981, in St. Peter's Square. And it reported that he initially thought his attacker was a member of the Italian terrorist organization the Red Brigades.
The book was written by Monsignor Slawomir Oder, the postulator, or main promoter, for John Paul's canonization cause and was released Tuesday. It was based on the testimony of the 114 witnesses and boxes of documentation Oder gathered on John Paul's life to support the case.
At a news conference Tuesday, Oder defended John Paul's practice of self-mortification, which some faithful use to remind them of the suffering of Jesus on the cross.
"It's an instrument of Christian perfection," Oder said, responding to questions about how such a practice could be condoned considering Catholic teaching holds that the human body is a gift from God.
In the book, Oder wrote that John Paul frequently denied himself food - especially during the holy season of Lent - and "frequently spent the night on the bare floor," messing up his bed in the morning so he wouldn't draw attention to his act of penitence.
"But it wasn't limited to this. As some members of his close entourage in Poland and in the Vatican were able to hear with their own ears, John Paul flagellated himself. In his armoire, amid all the vestments and hanging on a hanger, was a belt which he used as a whip and which he always brought to Castel Gandolfo," the papal retreat where John Paul vacationed each summer.
While there had long been rumours that John Paul practiced self-mortification, the book provides the first confirmation and concludes John Paul did so as an example of his faith.
Pope Benedict XVI put John Paul on the fast-track for possible sainthood weeks after his April 2, 2005 death by waiving the customary five-year waiting period before the process can begin. Last month, Benedict moved John Paul a step closer to possible beatification - the first major milestone in the process - by approving a decree on his "heroic virtues."
The Vatican must now confirm that a miracle attributed to John Paul's intercession occurred in order for him to be beatified - a step which many Vatican watchers have suggested may come as early as October.
Oder declined to speculate on any possible date, saying the miracle must still be confirmed.
The book publishes for the first time a never-delivered speech John Paul prepared for his weekly general audience Oct. 21, 1981, five months after the Turkish gunman, Ali Agca, shot him in St. Peter's Square.
Agca served a 19-year sentence in an Italian prison for shooting the pope, and earlier this month was released from a Turkish jail where he served a 10-year sentence for killing a Turkish journalist in 1979.
John Paul had publicly forgiven Agca on May 17, 1981 - four days after the assassination attempt. And he visited Agca in prison in 1983.
But five months after the attack, John Paul prepared a lengthy treatise on the power of forgiveness and the need for it in society, using his own experience as an example.
"The act of forgiveness is the first and fundamental condition so that we aren't divided and placed one against another like enemies," he wrote in what Oder called "an open letter" to Agca.
In the speech, he revealed that he while he had publicly forgiven Agca on May 17, "the possibility of pronouncing it before
in the ambulance that brought me from the Vatican to the Gemelli hospital where the first and decisive surgery was performed - I consider the fruit of a particular grace given to me by Jesus."
Oder speculates that John Paul decided against delivering the speech "out of prudence" for the ongoing criminal investigation into the shooting.
The book also reports for the first time that John Paul initially thought that the shooter had been a member of the Red Brigades, the radical leftist group that terrorized Italy in the 1970s and 80s. Some time before the shooting, the Italian secret service had reported to the Vatican a plot by the Red Brigades to kidnap the pope, the book said.
John Paul was apparently thinking of this when he told his secretary in the ambulance going to the hospital: "Just like Bachelet," an apparent reference to the assassination by the Red Brigades of the Catholic judge Vittorio Bachelet one year earlier, the book said.
The book also reports that John Paul first considered the possibility of resigning when he turned 75, the normal retirement age for bishops, going so far as to convene a group of close collaborators for an informal discussion on the topic.
He tasked then-Cardinal Joseph Ratzinger, head of the Vatican's orthodoxy office and future Pope Benedict XVI, to study the theological and historic issues implied in having an "emeritus pope."
In the end, John Paul left the question up to "providence" - he never resigned.
But he did outline on two separate occasions the criteria for which he would do so.
In 1994, he wrote what appeared to be a speech to be delivered to cardinals in which said he intended to resign "in the case of an illness determined to be incurable and which impedes the (sufficient) exercise of the function of the petrine ministry."
In a memo signed and dated five years earlier, on Feb. 15, 1989, he similarly wrote that if he was unable to sufficiently do his job because of an incurable illness, he would "renounce my sacred and canonical office" and leave it up to the top cardinals to carry out his wishes.
John Paul suffered from Parkinson's disease for many years before he eventually died of septic shock and cardiocirculatory collapse, preceded by heart and kidney failure brought on by a urinary tract infection.
Prior to his death, John Paul had been in and out of the hospital for two months and, by the end, had lost the ability to speak.
The book goes on sale in Italy on Wednesday. The publisher Rizzoli said there were no immediate plans for translations.