Pope Benedict XVI has published the third and last volume of his biography of Jesus Christ, a highly personal work written under the theologian pope's own name of Joseph Ratzinger.

The tome devoted to Jesus's childhood is being published in nine languages with a first edition of around a million copies, the Vatican said.

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.

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 Holy See.

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.

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 written by Ratzinger rather than Pope Benedict XVI, meaning that it is an academic work rather than dogma that cannot be contradicted.

The book is characterised by its "clarity and humility," Ravasi said.

'Jesus of Nazareth: The Infancy Narratives' goes on sale on Wednesday.

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.