The first three books, covering the reigns of Henry I and the first two Ottos (Otto I and Otto II) are largely based on previous chronicles most of which are still extant (e.g. Widukind of Corvey's "Res gestae Saxonicae", the "Annales Quedlinburgenses" and others); the fourth book, comprising the reign of Otto III contains much original matter; while the remaining four books, which describe the reign of Henry II to the year 1018, are the independent narrative of Thietmar and, besides being the principal source for Saxon history during the reign of Henry II, contain valuable information, not to be found elsewhere regarding the contemporary history and civilization of the Slavic tribes east of the river Elbe, as well as Poles, Hungarians and Bulgarians.