Tharwat El-Kharbawy's "Secret of the Temple", recently published by Nahdet Masr publishing house, has been dismissed by Brotherhood leaders as part of a smear campaign.
But its success points to a deep mistrust harboured by some Egyptians towards a once-outlawed movement that has moved to the heart of power since Hosni Mubarak was toppled and its candidate secured the presidency.
In its 12th print run since November, the book is being sold in upmarket shops and on street corners, pointing to a thirst for information about a group whose inner workings remain a mystery months after President Mohamed Morsi came to power.
Expelled from the group a decade ago, El-Kharabawy says he aims to expose dictatorship and extremism inside the Brotherhood. In the process, he has joined a media war being waged to shape views in Egypt's deeply polarised political landscape.
Asked to comment on the book, one senior Muslim Brotherhood leader dismissed its content as "fallacies". Another said that to comment on such a book would be a waste of time.
"I want to make all people know the reality about the Brotherhood," El-Kharabawy said in an interview with Reuters.
El-Kharabawy sees the way he was kicked out of the Brotherhood as an illustration of the group's authoritarian streak.
He was disciplined in 2001 at a "Brotherhood court" for publishing three articles that criticised the group for not engaging with other opposition parties - a criticism still levelled at the Brotherhood today. "The Brotherhood does not know the virtue of differences of opinion," he said.
Demonised for decades by Egypt's military-backed autocracy, the Brotherhood sees such attacks as propaganda concocted by opponents who have struggled to get organised and carve out their place in the new order.
But El-Kharabawy's arguments resonate among those Egyptians who believe the Brotherhood aims to subvert new freedoms for their own ends to set up a new Islamist autocracy - a view hardened late last year when Morsi unilaterally expanded his powers.
MORSI DEFENDS QUTB
El-Kharabawy has been extensively interviewed by independent Egyptian media that are broadly critical of the Brotherhood.
In his book, he explores the ideology of Morsi and the small group of leaders at the top of the movement, examining their devotion to Sayyid Qutb, a radical ideologue executed in 1966 for plotting to kill president Gamal Abdel Nasser.
Qutb, a Brotherhood leader, formulated some of the most radical ideas in political Islam. These included the idea that modern-day Muslim societies were living in a pre-Islamic state of ignorance. His most radical work, written while he was in prison, advocated violence to bring about change.
Morsi is on the record as defending Qutb as a thinker "who liberates the mind and touches the heart". In a 2009 talk show appearance posted on YouTube last year, Mursi said Qutb "finds the real vision of Islam that we are looking for".
Among Brotherhood watchers, it is no secret that the Brotherhood's current leadership were heavily influenced by Qutb, who also wrote more broadly on Islam.
But "trying to give the impression that Morsi is a Qutbist is an exaggeration" said Khalil El-Anani, an expert on Islamist movements. "Yes they are influenced by him in terms of the purity of ideas, but not in terms of believing in violence or judging people as non-believers," he said.
Brotherhood spokesman Ahmed Aref said the movement, like all groups, had rules that must be respected, adding that it was not the first time a member had left over the years and spoken out. "The difference this time is the media," he said.
A well-oiled campaign machine and grass-roots support base helped the Brotherhood sweep the first post-Mubarak parliamentary vote at the end of 2011, but the assembly was disbanded in June when Egypt's highest court declared the election rules unconstitutional.
Suspicion that the Brotherhood plans to dominate Egypt means the group may find it harder to win votes as fresh parliamentary elections near.
"They don't have people who can explain themselves in a good way, particularly those who talk to the Egyptian public," said Anani. "There is a huge gap of mistrust."