Hillary Rodham Clinton hasn’t shied away from the public stage in the months since her defeat in the 2016 election. She’s been giving speeches, sitting on panels, going to the theater, and taking selfies with supporters, and this fall, she’ll be publishing a book that promises to be “her most personal memoir yet,” according to her publisher, Simon & Schuster.

The book, titled “What Happened,” will offer an intimate view of what it was like for Mrs. Clinton to run as the first female presidential candidate from a major party in United States history, in an often vicious and turbulent campaign.

Ms. Clinton will explore the mistakes she made, what it was like to run against Donald J. Trump, the difficulties she has faced as a woman in politics, the role that Russian hacking played in the election, and how she recovered from the humiliating loss, according to a description of the book released by Simon & Schuster.

Most of all, though, the book will give insight into what she was thinking and feeling during one of the most dramatic campaigns in modern American history.

“In the past, for reasons I try to explain, I’ve often felt I had to be careful in public, like I was up on a wire without a net,” Mrs. Clinton writes in the book’s introduction. “Now I’m letting my guard down.”

“What Happened” is due out on Sept. 12.