The Cambridge History of the Byzantine Empire c.500–1492

The Roman Empire in the west fell in the mid 400s. It continued in the guise of the Eastern Roman Empire from the mid 400s to the mid 1400s . The Eastern Roman Empire was also known as the Byzantine Empire. The Roman Emperor Justinian was just after Rome fell in the late 5th century but was also one of the first of the Byzantine Emperors.

Monumental history book.