Former Vice President Kamala Harris revealed in her upcoming book, "107 Days," that then-President Joe Biden rattled her right before she went head-to-head with then-candidate Donald Trump on the debate stage.Biden reportedly called Harris as she sat in a hotel room preparing for the only debate of her abbreviated campaign. He apparently wanted to wish her luck and to scold her.The then-president said, "My brother called. Hes been talking to a group of real power brokers in Philly," according to an excerpt of the book in The Guardian. He then allegedly asked if Harris was familiar with several people related to the matter, which she was not.KAMALA HARRIS COMPLAINS ABOUT 'IMPOSSIBLE' LACK OF SUPPORT FROM BIDEN'S COMMS TEAM, INNER CIRCLE"His brother had told him that those guys were not going to support me because Id been saying bad things about him. He wasnt inclined to believe it, he claimed, but he thought I should know in case my team had been encouraging me to put daylight between the two of us," Harris wrote in the book, according to an excerpt of the book in The Guardian.Biden then went on to talk about his past debate performances, leaving Harris confused, "angry and disappointed," according to The Guardian. She was upset that her boss had called before a critical moment in her political career and made "it all about himself." Harris added that Biden was "distracting me with worry about hostile power-brokers in the biggest city of the most important state."Then-first gentleman Doug Emhoff apparently noticed his wife was in distress and advised her to "let it go" before facing off against Trump.HARRIS TAPS DEM EMAILS LISTS TO MARKET NEW BOOK, SPARKING PARTY NEUTRALITY CONCERNSWhile Harris avoided criticizing Biden during her campaign, she has used her upcoming book to shed light on the tensions between them as she took his place as the Democratic presidential nominee. Harris book is set to hit shelves on Sept. 23, but it has already sparked conversations about the 2024 election cycle.In another section, Harris said while "its Joe and Jills decision" became a mantra ahead of the 2024 election cycle, she said it was "recklessness," rather than "grace," according to an excerpt released by The Atlantic."'Its Joe and Jills decision.' We all said that, like a mantra, as if wed all been hypnotized. Was it grace, or was it recklessness? In retrospect, I think it was recklessness. The stakes were simply too high. This wasnt a choice that should have been left to an individuals ego, an individuals ambition. It should have been more than a personal decision," Harris wrote.Harris also revealed in her book that then-Transportation Secretary Pete Buttigieg was her "first choice" as running mate, not Minnesota Gov. Tim Walz. However, she said it was "too big of a risk" because the campaign was "already asking a lot of America: to accept a woman, a Black woman, a Black woman married to a Jewish man."Fox News Digital's Deirdre Heavey and Greg Norman contributed to this report.