In "Light of the World: The Pope, the Church and the Sign of the Times," Benedict condones the use of condoms in some instances.
The pope said that to prevent infection with HIV, for instance in the case of a male prostitute, a condom can be the first step in assuming moral responsibility.
He told German journalist Peter Seewald that a male prostitute using a condom is the "first step on the way to another, more humane sexuality."
It was the Vatican's first exception to a decades old ban on the use of condoms.
Christopher Lamb from Catholic Weekly, The Tablet, said the pope's use of the example of male prostitutes is meant to illustrate one of several cases where condom use is acceptable.
"It was an illustration, because he talks about cases, there being different cases where a condom can be used in certain circumstances. The key thing here is intention," Lamb said. "If the intention here is to preserve life, then that is OK. You can use the condom then."
Despite the gravity of the statement, there were some glaring omissions. The pope did not condone the use of condoms as birth control and he did not specifically mention female prostitutes.
The Vatican's position on contraception can be traced back more than 800 years when theologians argued against any form of sex not meant to produce children.
Father Joseph Fessio, editor and publisher of Ignatius Press and a friend of the pope's, said Benedict's comments were not meant to open up debate.
"He's not changing church teaching," Fessio said. "What he did say is if someone, a male prostitute for example, who decides to use a condom because he has some glimmer of moral responsibility to prevent disease, that could be a first step towards a greater moral attitude, better moral attitude to sexuality, but in itself the act is still immoral."
While some hope that the pope's words might lead to reform in the Catholic Church, Fessio says that isn't the case.
"I am sure we will not see a change either in this papacy or after this papacy," Fessio said. "The church's teaching is clear. The moral principles are quite solid and it's just a question of explaining it better to the people."
Still, saying that condom use to prevent the spread of a deadly disease shows moral responsibility is a first for any pope. UNAIDS, the United Nations HIV program, welcomed the comments as a "significant and positive step forward."
The church has faced intense criticism for its opposition to condoms, despite their proven use preventing the spread of HIV, particularly in Africa, where AIDS has killed tens of millions.
On a trip to Africa last year, Pope Benedict outraged health campaigners by saying condom use may actually worsen the spread of the disease.
The Vatican's hard line has sparked a backlash even from some of its own in Africa, including church-run health clinics that often ignore the teachings and distribute condoms.
Insiders say the pope is saying in public what has long been discussed in private inside the Vatican, where a paper has been considered for years allowing condom use between married couples, when one has AIDS.
The debate continues to sharply divide the Vatican but it could now play out in the open.