Article continues below advertisementBritney Spears is reportedly not happy about Kevin Federline's upcoming memoir.The "Toxic" singer's ex-husband - and the father of her two children, Sean Preston, 19 and Jayden James, 18 - recently announced that his memoir You Thought You Knew would be released on October 12 and is set to discuss his marriage to Spears from 2004-2007, as well as the pop icon's mental health decline that led to her 13-year abusive conservatorship.