Britney Spearsis not happy about ex-husbandKevin Federline's new memoir, a source tellsStar. The "Toxic" singer, 43, "is absolutely beside herself over Kevin doing this tell-all," says the source, and is "in fight mode." "She knew it was in the works but never thought he'd actually go through with it," the source shares. "Kevin's been teasing that he's finally telling his side of the story about what it was really like being married to her, and that's set off every alarm bell." The source says Spears — who was married to the former dancer, 47, from 2004 to 2007 — is "desperate to get her hands on a copy to see exactly what he's saying," but regardless, "she's convinced it's not going to be pretty." "She feels like, after everything she's been through, the last thing she needs is her ex dredging up the past for profit," the source claims. The source notes that because Spears was "fairly restrained" in what she wrote about Federline — with whom she shares sonsSean, 19, andJayden, 18 — in her 2023 memoir,The Woman in Me, he should follow suit. "If he decides to get too personal or twist things, she's ready to come out swinging with her own version of events," the source adds. "This has all stirred up a lot of painful memories for her and left her on edge." AsStarpreviously reported, Federline announced he is releasing a memoir titledYou Thought You Knewon October 21. "This book is extremely intimate and transparent," Federline said of the book, in a statement perRolling Stone. "I achieved my biggest dreams, dealt with crushing heartbreak, and endured constant ridicule, all while becoming the father my children needed as they experienced nonstop emotional turbulence." "If you've ever had questions," he added, "you'll find answers here."