Clinton gives readers a personal look in to her much-discussed -- and at times fraught -- marriage, reflecting on their more than 40 years together in terms that she has not used since her 2003 memoir "Living History."
"My marriage to Bill Clinton was the most consequential decision of my life," Clinton writes. "We've been married since 1975. We've had many, many more happy days than sad or angry ones."
The Clintons met at the Yale University Library in 1971 while both were attending law school at the prestigious school. Hillary Clinton rejected Bill Clinton the first two times he asked for her hand in marriage, writing in "What Happened" that she "knew marrying him would be like hitching a ride on a comet." They married in 1975.
Despite living much of their lives in the public eye, Clinton writes that she believes their marriage isn't "anyone's business. Public people should be allowed to have private lives, too."
But that hasn't been the case for the Clintons.
Trump made Bill Clinton's infidelity a campaign issue in 2016, even inviting women who accused the former president of inappropriate sexual behavior to an October debate. The Republican presidential nominee's allies have also discussed Monica Lewinsky, the intern with whom the former president had an affair in the 1990s.
Clinton addresses those "dark days" of their marriage in "What Happened," writing that the couple "certainly had" gone through low points.
"You know all about them -- and please consider for a moment what it would be like for the whole world to know about the worst moments in your relationship," she writes.
"There were times that I was deeply unsure about whether our marriage could or should survive. But on those days, I asked myself the questions that mattered to me: Do I still love him? And can I still be in this marriage without becoming unrecognizable to myself -- twisted by anger, resentment, or remoteness? The answers were always yes. So I kept going."
In "Living History," one of the first times Clinton fully discussed the Lewinsky scandal, the former first lady wrote that she could "hardly breathe" when she found out about the affair.
"I didn't know whether our marriage could -- or should -- survive such a stinging betrayal, but I knew I had to work through my feelings carefully, on my own timetable," she wrote, calling the decision to stay with Clinton one of "the most difficult decisions I have made in my life."
The passages about the Lewinsky scandal were some of the most personal reflections Clinton has publicly made. She discussed turning to her faith and feeling "unbearably" lonely in the aftermath.
In "What Happened," Clinton's reflections are equally as personal but focus almost entirely on the positives of their marriage.
"He has been my partner in life and my greatest champion since the moment we met," Clinton writes, reflecting on how he "never once suggested" that she put something on hold because it would interfere with his ambitions. "I still think he is one of the most handsome men I've ever known. ... I love him with my whole heart. That's more than enough to build life on."
Clinton writes about how she and her husband laid in bed after she conceded to Trump and just "stared at the ceiling."
"Bill took my hand, and we just lay there," she recalls.
And the former first lady also confronts all the worst public assumptions about their relationship.
"I know some people wonder why we're still together," she bluntly writes. "I heard it again on the 2016 campaign: that 'we must have an arrangement' (we do, it's called marriage); that I helped him become president and then stayed so he could help me become president (no); that we lead completely separate lives, and it's just a marriage on paper now (he is reading this over my shoulder in our kitchen with our dogs underfoot and in a minute he will reorganize our bookshelves for the millionth time, which means I will not be able to find any of my books, and once I learn the new system, he'll just redo it again, but I don't mind because he really loves to organize those bookshelves)."
The crescendo of Clinton's ode to her husband comes near the end of the book when she writes about how she worked hard on her May speech at Wellesley -- her alma mater -- and relied on edits from the former president.
"He stayed up to all hours reading the latest speech draft and dribbled on the top of the pages, 'H - I like this speech. Hope these suggestions help make it better -- wake me and we'll go over it -- I love you,' " she recalls, adding, "I thought for the 10 millionth time how glad I was that I had married my best friend and biggest cheerleader."
"And yes," she adds, "like always, his edit made my speech better."