Clinton joked about the gag, telling an audience here that she sent them so the Republican candidates -- who often knock her accomplishments leading the State Department -- could read about what she did.
"I have now sent each of them a copy of my book, 'Hard Choices,'" Clinton said to laughs. "There are so many of them, they could start a book club."
Clinton and her aides came up with the idea on September 18 -- shortly after a Republican debate -- while she was campaigning through New Hampshire and Maine, according to a spokesman.
After noting that each candidate went after her tenure at State, Clinton said, "Let's send them a copy of my book," her spokesman recalled.
The books went out to every Republican candidate except former Virginia Gov. Jim Gilmore, who wasn't at the most recent debate.
"I understand that you and your fellow Republican candidates for president were questioning my record of accomplishments at your last debate, so I thought you might enjoy reading my book, 'Hard Choices,'" Clinton's letter to her potential rivals said. "From working to restore America's standing in the world to bringing crippling sanctions to Iran to negotiating a ceasefire in Gaza, please enjoy all 596 pages of my time as secretary of state. With 15 candidates in the race, you've got enough people for a book club."
Clinton closed the letter: "With best wishes, I am sincerely yours, Hillary Rodham Clinton."
The letter was addressed to each candidate's formal name:
John E. Bush
Benjamin S. Carson, Sr., M.D.
Christopher J. Christie
Rafael E. Cruz
Cara C. Fiorina
Lindsey O. Graham
Michael D. Huckabee
John R. Kasich
George E. Pataki
Randal H. Paul, M.D.
Marco A. Rubio
Richard J. Santorum
Donald J. Trump
The news about Clinton's joke garnered a few reactions from the Republican field.
Sen. Ted Cruz offered to send Clinton a copy of his book, "A Time for Truth."
Kristy Campbell, a spokeswoman for Bush, said he hasn't received the book yet but added, "Governor Bush hopes Secretary Clinton will have an opportunity to read his e-book, Reply All, when it comes out in a few weeks. The book, based on Governor Bush's emails while in office, is a good lesson on the importance of transparency in government."
Louisiana Gov. Bobby Jindal offered Clinton a deal: He will read her book if she watches the controversial Planned Parenthood videos.
"Hard Choices" was published in 2014, more than a year after Clinton finished her four-year term as secretary of state. The book details Clinton's time as secretary of state, including the controversial 2012 terrorist attack in Benghazi and the mission that killed Osama bin Laden.
After the book was published, Clinton embarked on a nationwide book tour that many saw as the start of her presidential campaign.