Von Domazewski, _Römische Kaiser_, ii, pp. 168–318; Gibbon, E., _Decline
and Fall of the Roman Empire_, ed. Bury, i, chs. i–xii; Niese,
_Geschichte_, pp. 331–376; Schiller, _Römische Kaiserzeit_, vol. i, ch.
ii, §§ 57–59; chs. iii–iv; Stuart Jones, chs. v–ix. More special:
Gregorovius, F., _The Emperor Hadrian_; Platnauer, M., _The Life and Reign
of Septimius Severus_; J. Stuart Hay, _The Amazing Emperor Heliogabalus_.