Now, they've turned their pens on Ronald Reagan with "Killing Reagan," and like several of the prior books in the "killing series," it's getting plenty of criticism.
Of course, no one actually killed President Reagan. John W. Hinckley Jr. tried to assassinate him on March 30, 1981, shooting him in the chest outside the Washington Hilton, but Reagan survived, completed his terms in office, and went on to live to the age of 93.
Rather, "Killing Reagan" explores how dementia gradually took over the life of our 40th president.
The twist: Though his diagnosis was officially announced in 1994, in their book, Mr. O'Reilly and Mr. Dugard claim that Reagan showed signs of Alzheimer's during his presidency.
Throughout much of the book, Reagan is depicted as aloof, adrift, and confused, out of touch with the day-to-day workings of the White House.
"He delegates much power to Nancy," the authors write of Reagan in 1987. "Occasionally, he avoids the Oval Office altogether, spending hours during the day watching television reruns in the upstairs residence. Even more troubling, it is no longer given that the president will take the time to read important policy papers."
They also depict Reagan as a womanizer who had multiple extramarital affairs in Hollywood, an aloof father who spent little time with his children, and a contrarian husband who frequently fought with his second wife, Nancy.
Considering Reagan's iconic status among conservatives and O'Reilly's prominence in conservative circles, "Killing Reagan" is a surprisingly critical appraisal.
And, considering the criticism leveled, the 300-plus-page tome is also surprisingly thinly researched, with only two-and-a-half pages of sources.
Which is one reason it's been roundly condemned by Reagan experts.
"'Killing Reagan' is historical fiction instead of biography," declared Real Clear Politics.
"Another Reagan senility myth," said The Washington Times.
And a group of four Reagan scholars and experts who, between them, have written 19 books about the former president, recently discredited the book in an editorial for The Washington Post.
"Killing Reagan," they write, "restates old claims and rumors, virtually all of which have been discredited by the historical record."
In addition to the a litany of other details, they say the central theme - that Reagan was losing his faculties even in office - is wrong.
"It speaks volumes that none of the hundreds of former Reagan White House staffers has stepped forward to corroborate the story," the Reagan experts write. "Reagan’s national security adviser, Richard V. Allen, told us flatly that “Killing Reagan” is “garbage.”"
They, and other sources also point to the book's flimsy sourcing: no endnotes, no bibliography, no long list of interviewees, "only a smattering of footnotes."
On his Fox News show and in interviews, O'Reilly has responded to the criticism, saying Reagan loyalists are trying to stifle the truth, and that he and his co-author are regularly threatened over the books in their "killing" series.
Even before the book was completed, former California Gov. Pete Wilson and former Reagan White House staffer Christopher Cox called Dugard and warned him "not to say anything negative about Mr. Reagan," O'Reilly said. "The Reagan guardians even went to my bosses to try and spike the book."
That hasn't changed the opinion of some reviewers.
As USA Today wrote, "[T]his book may be the final sign that it's time for O'Reilly and Dugard to kill off this series."