Hillary ClintonHillary Diane Rodham ClintonLewandowski says 'womp womp' at story of young girl being separated from mother at border Giuliani: FBI asked me about tease of a 'surprise' before election Republicans tear into IG finding on Clinton probe MORE describes her nomination as secretary of State under President Obama after the two battled in the 2008 primary as "unexpected" in an excerpt of her new book, Hard Choices

She said the years written about were a personal journey for her, from the "painful end of the 2008 campaign to an unexpected partnership and friendship with my former rival Barack Obama."

In the author's note released Tuesday, Clinton said many people asked "Are you out of your mind?" when she married Bill Clinton and moved to Arkansas decades ago. She said she heard similar questions when she took the job at the State Department in 2009.

“When I chose to leave a career as a young lawyer in Washington to move to Arkansas to marry Bill and start a family, my friends asked, ‘Are you out of your mind?’ I heard similar questions when I took on health care reform as First Lady, ran for office myself, and accepted President Barack Obama’s offer to represent our country as Secretary of State," she writes in the book. 

She said the book is about hard choices made by her, President Obama and other leaders. She said it is for those who want to know what American stood for and how the Obama administration confronted great challenges. 

"While my views and experiences will surely be scrutinized by followers of Washington’s long-running soap opera — who took what side, who opposed whom, who was up and who was down — I didn’t write this book for them," she said. 

Clinton is seen as the front-runner for the Democratic nomination for president in 2016 if she decides to make a bid. She has previously said she will make a decision on a White House run later this year. 

The book is set for publication on June 10. It is expected to span the timeframe from the conclusion of her failed White House bid in 2008 through her time at the State Department. 

“I’ve served our country in one way or another for decades," she writes. "Yet during my years as Secretary of State, I learned even more about our exceptional strengths and what it will take for us to compete and thrive at home and abroad."

She said her faith in the future of the United States has never been stronger, even though talk of the country's decline is "commonplace."

"While there are few problems in today’s world that the United States can solve alone, there are even fewer that can be solved without the United States. Everything that I have done and seen has convinced me that America remains the 'indispensable nation,' " she writes. 

In a separate passage, she described the effort to hunt down and kill Osama Bin Laden in 2011 "as crisp and courageous a display of leadership as I've ever seen."

"The President’s top advisors were divided," she writes. "The intelligence was compelling, but far from definitive. The risks of failure were daunting."

—Updated 8:50 a.m.