Parthians and Romans
In 116 AD, the Roman emperor Trajan invaded the Parthian empire and conquered all the way to Babylon. The Parthians were not very strong at this time, because of civil wars, so they couldn't fight back very well. In 117 AD, just a year later, Trajan's successor Hadrian gave up most of the land that Trajan had conquered, but he made the Parthians pay the Romans a lot of money.
By 161 AD, forty years later, the Parthians were united again under the strong king Vologeses III. When the Roman emperor Antoninus Pius died, the Parthians thought this would be a good chance to attack the Romans. So they did. After a big war, however, the Parthians lost again. This may be partly because of a great plague (a disease that many people got, and many people died of) that attacked them at this time. (We aren't sure what the disease was, but it may have been smallpox). Then in the 220s AD, new rulers tossed out the last weak Parthian kings. These new rulers called themselves the Sassanids.
Ancient Persia, by Don Nardo (2003). Good for reports.
Parthian Art, by Malcolm Colledge (1977). Not very easy to get anymore, but it's the classic book for the Parthians - it has a lot more than just the art.
Ancient Persia, by Josef Wiesehofer (2001). Includes the Parthians and the Sassanians.
The Cambridge History of Iran: Volume 3, The Seleucid, Parthian and Sasanid Periods, Part 2: Seleucid Parthian, edited by E. Yarshater (reprinted 1983). Expensive but complete.