What has Benedict said?
In the book, titled "From the Depths of Our Hearts," the 92-year-old pontiff argues in favor of the centuries-old tradition of celibacy within the church, defending the ability to "put oneself completely at the disposition of the Lord" as a criterion for those wishing to be ordained as priests.
Benedict also wrote that he believes celibacy carries "great significance" and is "truly essential" as a priest's path to God becomes the foundation of his life.
"The call to follow Jesus is not possible without this sign of freedom and of renunciation of all commitments," he wrote.
"Celibacy must penetrate, with its requirements, all of the attitudes of existence."
What has caused the controversy?
The proposal, which is pending approval by Francis, would be a historic change to the church's centuries-old tradition of unmarried priests, but critics fear that if approved, such a move could leader to a wider dissolution of the discipline of celibacy around the world.
Approved by Catholic bishops by a vote of 128-41, the proposal applies only to some churches in the Amazon region, including parts of Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname and Venezuela.
Allowing married men to become priests is not the same thing as allowing priests to marry, so the change would not affect the rule of celibacy for Catholic priests, who are not allowed to marry.
Still, the possibility caused outrage among conservative groups in October, who protested with prayer vigils after American Cardinal Raymond Burke and Bishop Athanasius Schneider of Kazakhstan called for "a crusade of prayer and fasting" for 40 days throughout the meeting.
There was no official conclusion to the summit. However, the final documentation that emerged from the meeting strongly recommended the change.
Francis is expected to publish his conclusions on the proposal in the coming months.
Benedict retired in 2013 and has largely stayed away from sharing his thoughts on any of the measures adopted by his successor or any of the issues surrounding the church.
When he retired in 2013, he vowed to remain "hidden from the world."
Reactions to the retired pontiff breaking that silence now are split along largely predictable political lines.
Supporters of Francis, such as British biographer Austen Ivereigh, have suggested it's inappropriate for a former pope to comment on a decision currently being weighed by the sitting pope.
Critics of Francis, such as Italian commentator Sandro Magister, have said that given the gravity of the issue, Benedict was not only fully in line to speak out, but that he was actually obligated to do so.
It remains to be seen whether Benedict and Sarah's comments will have any impact on Francis' decision. When he faced a choice in 2016 about opening Communion to divorced and civilly remarried Catholics, Francis met with significant internal opposition, but in the end the pontiff decided to forge ahead.
It's also possible that the controversy may reopen debate about the institution of a "pope emeritus."
When Benedict became the first pope in centuries to resign in 2013, decisions about his title, living arrangements and role were made hurriedly. At the time, some experts believed he should go back to being simply "Cardinal Joseph Ratzinger," his previous title and given name, to emphasize that he no longer held any papal authority and to reduce the chance he might be seen as a rival point of reference to his successor.
What has Francis said on the matter?
The present pope has said in the past that he did not agree with allowing optional celibacy.
On a flight to Rome from Panama in January 2019, Francis told journalists he believes in celibacy, but that the question should be studied.
When asked by a journalist if he would allow married men to become priests, Francis said: "Personally, I believe that celibacy is a gift to the Church. Secondly, I'm not in agreement with allowing optional celibacy. No.
"There could only be a possibility in these far, far away places, I think about the islands in the Pacific, but it's something to think about, when there's a pastoral need, there the shepherd has to think about the faithful. There's a book by Father Lobinger. He's interesting, this is a matter of discussion among theologians. It's not for me to decide. My decision is, optional celibacy before the diaconate, no. I will not do this. I don't feel like standing in front of God with this decision."
The Vatican told CNN on Monday that it was not currently commenting on the matter.