Modern Hagiography
Lives of the Saints by Alban Butler.
Lives of the Saints by Sabine Baring-Gould.
Our Birthday Bouquet Culled from the Shrines of the Saints and the Gardens of the Poets by Eleanor C. Donnelly.
Pictorial Half Hours with the Saints by Auguste François Lecanu.
Medieval Hagiography
Liber pontificalis (free to borrow with registration)
Writings by Pope Gregory II
Art & Architecture
2589 Painting by the Master of the Liesborn School of Westphalia (but no evidence that it depicts Saint Gregory II, nor even Saint Gregory the Great; more about the full altarpiece)
Nihil sub sole novum 