On al-Tabari and other Abbasid writers as sources for history, you're definitely going to want to read Tayeb El-Hibri's Reinterpreting Islamic Historiography: Harun al-Rashid and the Narrative of the Abbasid Caliphate (1999) which powerfully argues that the Abbasid chronicle tradition was much more concerned with forming the Abbasid story into a series of religious and moral fables rather than a simple presentation of historical fact.