The history of Roman Imperial architecture is one of the interaction of two dominant in Rome itself the emergence of a new architecture based on the use of a revolutionary new material, Roman concrete; and in the provinces, the development of interrelated but distinctive Romano-provicial schools. The metropolitan school, exemplified in the Pantheon, the Imperial Baths, and the apartment houses of Ostia, constitutes Rome's great original contribution. The role of the provinces ranged from the preservation of a lively Hellenistic tradition to the assimilation of ideas from the east and from the military frontiers. It was―finally―Late Roman architecture that transmitted the heritage of Greece and Rome to the medieval world.
John Bryan Ward-Perkins, CMG, CBE, FBA was a British architectural historian and archaeologist; he was director of the British School at Rome from 1946 to 1974.
Ward-Perkins is the standard handbook for advanced introductions to Roman Imperial Architecture - which doesn't make for exceptionally entertaining reading. The book attacks and summarizes the major monuments, technological developments and stylistic changes through time and across the Empire. There's a fairly extensive bibliography, although it is a little outdated now. This book is a great place to start when thinking about architecture. There aren't many technical descriptions (engineering, chemistry of concretes etc), which may be a downside to those who are interested in considering how things were built and what the state of technology was in antiquity. There is minimal description of decorative finishing or the social/political/religious implications of the changing styles, but this book doesn't pretend to be about social commentary. What it intends to do it does very well, namely, it describes the developments in architectural style in the Roman Empire. There are many lovely pictures, although at times there are relatively lengthy descriptions of buildings not presented in the illustrations.