In her most detailed public comments about what happened during the second presidential debate, Hillary Clinton said her "skin crawled" as Donald Trump loomed behind her on the stage in St. Louis last October.

The comments are included in her new book, "What Happened," which she called an attempt to "pull back the curtain" on her losing bid for the presidency. Some of the moments during the campaign, she said, "baffled" her.

The book comes out Sept. 12, but audio excerpts, read by Clinton herself, were played on MSNBC on Wednesday morning.

In the recording, Clinton noted that she wrote about moments from the campaign that she wanted to remember forever - as well as others she wished she could "go back and do over."

The moment from the debate appeared to fall into the latter category.

"This is not okay, I thought," Clinton said, reading from her book. "It was the second presidential debate and Donald Trump was looming behind me. Two days before, the world heard him brag about groping woman. Now we were on a small stage and no matter where I walked, he followed me closely, staring at me, making faces.

"It was incredibly uncomfortable. He was literally breathing down my neck. My skin crawled. It was one of those moments where you wish you could hit pause and ask everyone watching, 'Well, what would you do?' Do you stay calm, keep smiling and carry on as if he weren't repeatedly invading your space? Or do you turn, look him in the eye and say loudly and clearly, 'Back up, you creep. Get away from me. I know you love to intimidate women, but you can't intimidate me, so back up.'"

Clinton said the book was not a comprehensive account of the 2016 race - and that "What Happened" was difficult to write.

"Every day that I was a candidate for president, I knew that millions of people were counting on me, and I couldn't bear the idea of letting them down - but I did," she said in the audio excerpt. "I couldn't get the job done, and I'll have to live with that for the rest of my life."

Simon & Schuster, the book's publisher, says "What Happened" is Clinton's "most personal memoir yet."

"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," she writes in the book's introduction. "Now I'm letting my guard down."