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," Claudio Rossini, director of the Libreria Editrice Vaticana bookshop next to St Peter's Square told ANSA.
Some attribute the success of the Pope’s letter to the fact that it is his first, coupled with his reputation as a scholar and Theological expert. 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.
In the letter, Pope Benedict reflects upon the concepts of eros, agape, and philia - the three ancient classifications of love - and their relationship with the teachings of Jesus. The Holy Father calls all men and women to seek the divine origins of each type of love.
“I wanted here,” the Pope says in the document, “to clarify some essential facts concerning the love which God mysteriously and gratuitously offers to man, together with the intrinsic link between that Love and the reality of human love.”
Its been just over a year since its release, yet Pope Benedict XVI's first Encyclical letter continues to be a best seller. According to the Rome-based ANSA news agency the Pontiff’s profound discussion of human and divine love is proving to be one of the most commercially successful doctrinal tracts ever written by a Pope.