Pope Benedict XVI's weighty discussion of human and divine love, published almost exactly a year ago, is proving to be one of the most commercially successful doctrinal tracts ever written by a pontiff.
The 72-page document, Deus Caritas Est (God is Love), has been reprinted three times in the German pope's own language, three times in Spanish and sold almost 1.5 million copies in Italian.
Its success has also meant that, for the first time in modern history, the Latin version of a papal document has had to be reprinted. The initial run of 1,000 copies sold out in two months.
"Even now, a year after its publication, we're still shifting a few copies every day in the various languages," said Claudio Rossini, director of the Libreria Editrice Vaticana bookshop next to St Peter's Square.
Benedict's first 'encyclical letter', a form of papal writing aimed at a non-expert readership, has even been translated into Russian and Chinese in a bid to reach out to Catholics and non-Catholics there.
In the document, the pope says that in today's world the true nature of love between men and women needs to be clarified. He says that erotic love can lead to degradation unless it is united with a spiritual love.
Many observers were surprised by the document's tone and the understanding it seemed to show of what eros was all about. British daily The Times said the text was a "startling revelation," commenting that "The language itself verges at times on the erotic".
According to Vatican-watchers, the encyclical has sold well partly because of the curiosity aroused by the first major piece of writing by a new pope.
There was particular interest in this case because Benedict was well known before his election as head of the Catholic Church's doctrinal watchdog, a role which gave him an aura of strictness and austerity.
But Prof. Ilaria Morali, who teaches dogmatic theology at the Pontifical Gregorian University, believes the subject matter of the encyclical has also played a role.
"Love is something that everyone is interested in. People know that here at least the subject will not be treated lightly or in a banal way," she said.
Other observers have pointed to the fact that Benedict has made a point of limiting his official pronouncements on weighty subjects, so that each one will have maximum impact.
Before he became pope, the then Cardinal Joseph Ratzinger was widely respected as one of the Church's top theologians. Leading members of the Anglican church and other Christian denominations often acknowledge his intellectual stature.
