Einhard was employed by Charlemagne as a court historian. At the request of Charlemagne's son and successor Louis the Pious, he wrote a biography of Charlemagne, the Vita Karoli Magni or Life of Charlemagne (c. 817–830), which provides much direct information about Charlemagne's life and character. In composing this he made full use of the Frankish Royal annals. Einhard's literary model was the classical work of the Roman historian Suetonius, the Lives of the Caesars. (summary adapted from Wikipedia by K. Merline)
