For the Sassanian Empire Touraj Daryaee's Sasanian Persia: The Rise and Fall of an Empire is a really good book which gives an overview of various aspects of Sassanian Society, including languages, religion, economy and administration. I also personally found it very readable, coming to the subject from a fairly low knowledge base.

Not a book, but Richard Bulliet has an iTunes U course up called, "The history of Iran to the Safavid Period" with portions devoted to the Parthian/Sassanians. The lectures are tremendous, it would be a great addition to provide a greater time-frame and context to any books you choose to read.