around a million copies, the Vatican said on Tuesday.
Its 147 pages are the result of years of work that began in 2003 when Ratzinger was still head of the Congregation for the Doctrine of the Faith.
"Jesus of Nazareth: The Infancy Narratives", which goes on sale in 50 countries on Wednesday, just over a month before Christmas, serves up some home truths about prevalent Christian myths.
The son of God was actually born several years earlier than stipulated in the Christian calendar because the monk who drew it up got his maths wrong -- and there were no donkeys or oxen present in the stable where Christ was born.
"The gospel makes no mention of animals," Benedict writes, but reassures those preparing their Nativity scenes in the run-up to Christmas that the beasts have become part of Christian iconography and are now accepted.
Benedict also intervenes in the debate over where Jesus was born, rejecting arguments by some scholars that he was born in Nazareth rather than Bethlehem.
The pope has devoted "all his free time to bring to fruition this project, which he wanted and loved," Vatican spokesman Federico Lombardi told hundreds of journalists and clergymen assembled in a large hall in the Vatican.
The book was what the pope "has most at heart" and matured as part of "a long internal journey" from when he first entered the clergy in Germany.
Before being elected pope, the scholarly Ratzinger reportedly tried several times to retire from the Congregation in order to dedicate himself full time to his writings, but his proffered resignation was repeatedly refused.
Benedict has also said he would not hesitate to become the first head of the Roman Catholic Church to resign willingly in more than 700 years if necessary -- and critics have accused him of dedicating more time to his books than to his papal tasks.
Cardinal Gianfranco Ravasi, head of the Pontifical Council for Culture, said the volume focuses on 180 verses from the Bibles of Matthew and John.
The book is "a gateway into a palace where we can already hear voices coming from chambers inside, particularly the question that Pontius Pilate asked Jesus at his trial 33 years later: 'Who are you?'" Ravasi said.
The book is signed "Joseph Ratzinger Benedict XVI", meaning that it is an academic work rather than papal dogma that cannot be contradicted.
The book is characterised by its "clarity and humility", Ravasi said.
The first two volumes of the biography "From the Baptism in the Jordan to the Transfiguration" and "From the Entrance Into Jerusalem to the Resurrection" were published in 2007 and 2011.