Von Domazewski, _Römische Kaiser_, i, pp. 251–305; ii, pp. 1–158; Niese,
_Geschichte_, pp. 304–331; Pelham, _Essays_, iii, _The Early Roman
Emperors_; Schiller, _Römische Kaiserzeit_, ii, ch. i, §§ 32–44; ch. ii,
§§ 53–56; Stuart Jones, _Roman Empire_, chs. ii–iv. More special: for
Caligula, H. Willrich, _Beiträge zur alten Geschichte_, 1903, pp. 85 ff.,
288 ff., 395 ff.; for Nero, Henderson, B., _The Life and Principate of the
Emperor Nero_; for the period 68–69, Hardy, G. S., _Studies in Roman
History_, 2nd ser., _The Four Emperors’ Year_; Henderson, _Civil War and
Rebellion in the Roman Empire_.