12 April – Saint Julius

Modern Hagiography

Roman Martyrology

Sanctorale Catholicum

Lives of the Saints by Alban Butler.

Lives of the Saints by Sabine Baring-Gould.

Character Calendar

Pictorial Lives of the Saints 

Medieval Hagiography

BHL

Acta Sanctorum

Liber pontificalis

Writings of Pope Julius

Documenta Catholica Omnia

His correspondence (not in the public domain)