This book tells only half the story, the first half. To complement it one has to read what led to Pope Benedict XVI's absolutely surprising resignation in 2013, especially in the book by Gianluigi Nuzzi on the filching of documents from the Pope's own desk that ultimately seems to have led the Pope to realise he could not really act like a proper Pope.

Yet what led to this conclusion is all, mostly, in this book which appeared months before the resignation but at the end seems to forecast it.

He should never have become Pope, Politi begins. Not because of what happened later but because Ratzinger as a cardinal was a deeply divisive person and the way popes are chosen is supposed to avoid putting divisive figures at the top.

Ratzinger began life as the expert of Cardinal Frings in the first sessions of the Vatican Council, well-known for berating the Curia. Ratzinger thus joined the topmost levels of moderately progressive theologians who guided the bishops in their conclusions.

But as the impact of the Council led to an earthquake inside the Church, Ratzinger became aghast at what he saw as the destruction of the Church he knew in 1973 and from progressive he became a pillar of orthodoxy.

He was appointed as Archbishop of Munich in his native Germany and it was here that he became, at least in one case, too lenient in a case of a priest later found guilty of sex crimes with an underage person. An independent investigation in recent days found him guilty. After some days Ratzinger has admitted his fault and asked pardon.

From Munich he was recalled to the Vatican and appointed as head of the Holy Office. It was here that he earned his fame as "the dog of God" and hammer of all liberals.

When John Paul II died after a long sickness, Ratzinger became the most likely successor. He was the main celebrant at the Pope's funeral and at the Mass that introduced the Conclave.

It was to be one of the shortest conclaves. Politi tracks the voting and shows how in the first ballot Ratzinger was already the front runner with 47 votes, followed surprisingly by the Argentinian Bergoglio with 10.

The next two ballots saw the same pattern with no third candidate emerging. Crunch time came around lunch on the second day. On the fourth ballot Ratzinger clinched the two thirds necessary and became Pope Benedict XVI.

After the pope's honeymoon the problems began to appear.

The first problem was the scholarly conference at the University of Regensburg where the Pope quoted a harsh condemnation of Mohammed, which led to fierce rebuttals by the Islamic world.

Next he reopened the doors to the Latin Mass to placate the arch-conservatives in the Church, angering those in favour of liturgical reforms.

Next he managed to anger the Jews. Coming from a nation that had much to ask pardon for, Benedict still managed to rile the Jews. First he apparently forgot to mention the Holocaust in a speech until his aides noticed it and got it corrected. Then, in another speech he seemed to say that Nazism was an aberration of some rather than the expression of an entire people. And he also did not mention the culpable inaction by the Catholics and their hierarchy. He even defended and beatified Pope Pius XII, who is widely claimed to have been less than strong in condemning Nazism.

Two issues then became inextricably linked. While Benedict was spearheading reconciliation with followers of arch-conservative Marcel Lefebvre it was found out that one of these bishops, Richard Williamson, was a convinced denier of the Holocaust. Once again, the Jewish world erupted in protest.

Next come two instances where appointments by the Pope had to be withdrawn due to the opposition they encountered. First there was the case of Gerhard Wagner as auxiliary bishop of Linz in Austria, an out and out conservative who considered Harry Potter as "work of the devil", the 2004 tsunami a divine punishment and the Katrina disaster likewise. He also claimed homosexuality a sickness that could be cured.

The Catholics of Austria had had other clashes with the Vatican when Pope John Paul II imposed a fervent conservative Hans Hermann Grover and made him a cardinal only to find later on that he had a track record of molesting junior seminarians. In the case of Wagner the pressures by the entire body of bishops and the laity of the area brought about the decision by Wagner to withdraw his appointment especially when it was found out his name did not feature in the usual three names that the Nuncio forwards to the Vatican.

Even worse was the case that happened in Poland. In December 2006 Benedict chose as Archbishop of Warsaw Stanislaus Wielgus, former bishop of Plock. But on 20 December the Gazeta Polska, a right-wing paper, claimed that Wielgus, when philosophy professor, had collaborated with the Communist authorities and the secret service. At first the Vatican supported the Pope's choice. After a while, Wielgus began to admit. He had entertained relations with the secret service to obtain visas to go abroad.

On 4 January, with the inaugural day just three days away, the daily paper Rzeczpospolita published the entire file on Wielgus, codename "Grey". He was a conscious collaborator of the regime. While the Vatican continued to say the ceremony was going to be held, many of the invitees began to desert the ceremony. When he arrived at the cathedral, Wielgus, white-faced, did not take the archbishop's throne. Instead, standing along the other bishops he announced his resignation.

Then of course there have been innumerable controversies regarding sex in the widest sense of the word. From his throwaway comment when on the plane, taking him to Africa, that condoms actually increase the risks of Aids...

To the tormented issue of clerical sex abuse on minors. 2010 was in this sense an "Annus Horribilis". It was in his visit in Malta in April 2010 that Benedict had one of his first meetings with victims of clergy abuse.

And even before Ratzinger became Pope, Archbishop Charles Scicluna was travelling the world investigating abuses even as John Paul II lay dying. His investigation continued under Benedict.

Then came his resignation, outside the scope of this book. The picture one gets at the end is of a gentle soul, a fine theologian, who charmed his listeners when he lectured in Paris and in London but who on the whole could only be a part-time Pope.