Buhari's authorised biography revealed that the stability of the nation's political system could be at stake if the EFCC goes after ex-Presidents.
The book, written by Professor John Paden, revealed that Buhari is in possession of implicating letters written by former President Goodluck Jonathan.
Jonathan, in the letter, was said to have requested "off-budget funds" - funds not included in the Federal Government budget.
In chapter 20 of the book, titled 'Corruption and law in military procurement,' the author captured how PDP leaders diverted and looted the nation's funds.
According to Paden, the purpose of Buhari's anti-corruption war is not to jail former top government officials but to retrieve the stolen funds.
He said if the Economic Financial Crimes Commission (EFCC) goes after Jonathan or other former Presidents, or military leaders, the stability of Nigeria's political system would be at stake.
Paden, however, said such former leaders, in exchange for immunity, could help the government identify patterns and sources of corruption.
This is believed to be the reason the EFCC has not gone after Jonathan and other former leaders yet.
The anti-graft agency has, since the anti-corruption war took off, limited its drag net to the possible beneficiaries of looted funds - PDP Chieftains and other top officials who served under Jonathan.
The author wrote: "The fact that Buhari was enlisting the help of the international community in the probes lent weight to the seriousness of his effort – and also meant that alleged offenders had nowhere to hide.
"Would the trail lead to former President Jonathan himself? As of the early months of 2016, it appeared that the EFCC was not going after Jonathan. Nor was it going after former President Obasanjo.
"The question of the stability of the entire political system seemed at stake.
"In addition, a number of senior military officers, who had served as Heads of State – from Babangida to Abubakar – seemed off-limits.
"Indeed, rumours swirled that if the probes went after senior officers, they might push back because they had extensive networks in the active military services.
"At the same time, the knowledge such heavyweights possessed could well be traded for immunity and would help to illuminate the patterns and sources of corruption.
"Buhari had letters in his possession showing Jonathan’s requests for off-budget funds. But Buhari’s larger purpose was not to put former high-level officials in jail.
"Rather, it was to retrieve stolen funds and to change the political culture of the country."
On the 2016 budget padding scandal, the book revealed that a total of 184 civil servants were penalised for their different roles in the padding, while 22 top officials among the number were dismissed from service.