私家 is a noun that literally means 'one's own family', but in modern Chinese it is most often used as an attributive modifier in compounds to indicate something privately owned or operated, as opposed to public (公家). Common compounds include 私家车 (private car) and 私家侦探 (private detective). It is not used for 'private' in the sense of personal privacy (that would be 私下 or 私人). The second sense 'private writings about history' is rare and literary.