I'm an early modernist but have been reading a lot in late antiquity for - couple of years. I'm interested in what the last 'pagan' Persian empire was like. Thanks!
The most recent study is P. Pourshariati's Decline and Fall of the Sasanian Empire (2008), arguing that rather than being a strong centralised state, the Sasanian Empire was very decentralised, more akin to a confederacy of multiple ethnic groups than the bureaucratic Roman state. I'm rather convinced by this thesis, but it is still worth looking up the chapters on Sasanian Persia in the Cambridge History of Iran (1983) or J. Howard-Johnston's comparative study of the empire and Rome in The Byzantine and Early Islamic Near East III (1995) to get a sense of the older view of how the Sasanians ruled their empire.
M. Canepa's The Two Eyes of the World: Competition and Exchange in the Art and Ritual of Kingship between Rome and Sasanian Iran (2009) is a good overview of the relationship between the Sasanians and the Romans, likewise for E. Fowden's The Barbarian Plain: St Sergius between Rome and Iran (1999) and G. Fowden's Empire to Commonwealth: Consequences of Monotheism in Late Antiquity (1993). The former is a very specific study of a local saint's cult between the two empires, whilst the latter is a much larger study that puts the two empires in their monotheistic context - Sasanian Persia was very similar to Christian Rome and the Islamic Caliphate in terms of its political culture/aspirations!
Just to tack on a few other suggestions:
Rahim Sheyegan, Arsacids and Sasanians and Aspects of History and Epic in Ancient Iran; studies of the relationship between Sasanian political ideology and earlier Iranian states such as the Arsacids and Achaemenians. These take more of a diachronic view as opposed to the comparative view of Canepa. Shayegan also claims to be working on a large-scale history of Sasanian Iran, which will presumably come out at some point if he finishes it.
Garth Fowden, Qusayr Amra; not specifically on the Sasanians but addresses the cultural influence of late antique Iran on the early Islamic caliphate so it may be of some interest.
One of the best books I ever read on Sassanian, Parthian and Achaemenid Persia was Shadows in the Desert, by Kaveh Farrokh. The text is about the military capabilities of each of the three civilizations, and can be found here:
http://www.amazon.com/Shadows-Desert-Ancient-General-Military/dp/1846031087
Another good book is Sasanian Iran (224-651 CE): Portrait of a Late Antique Empire, by Touraj Daryaee. It offers a good, detailed examination of Sassanian legal, military, political, religious and social organization. It is also available on Amazon:
One of the latest books is by Daryaee, and is called Sasanian Persia: The Rise and Fall of an Empire: