The Divine Comedy by Dante Alighieri (c. 1308–1320) is the obvious starting point — the poet who died in Ravenna wrote his masterpiece elsewhere, but parts of the Paradiso were completed here. Reading even the first cantica (Inferno) before visiting Dante's tomb gives the visit a completely different character. The Clive James translation (2013) is the most readable modern English version.
Justinian's Flea by William Rosen (2007) covers the age of Justinian — the Byzantine emperor whose mosaics dominate San Vitale — through the lens of the bubonic plague that struck the empire in 541 AD. It is a vivid account of the world that produced Ravenna's greatest art, and explains why Justinian was both so powerful and so anxious to project his authority through images.
The Pillars of the Earth by Ken Follett (1989) is not set in Ravenna, but it is the best accessible account of how medieval church architecture was conceived, funded, and built — context that makes visiting any major early medieval church more rewarding. Read it on the train south.