I am reading this book and the author keeps referring to people as Iranian when I would think to refer to them as Persian (for example, Zoroastrianism brought to Xinjiang by Iranians, a case brought to the Tang court by an Iranian merchant {this example is in the introduction on the "look inside" on Amazon}, etc). What is the difference between the two terms, and why would the author make this choice of terminology?
In modern academic history writing, the term "Iranian" has a broader meaning than "Persian." Iranian^1 refers to peoples who spoke Iranian languages. Persian is an Iranian language, as are Kurdish, Yaghnobi, Balochi, Ossetian, and so on. Persian specifically refers to a language which originated in Fars, the province of the capital of the Achaemenid (Persian) Empire. You could say Persians are Iranians but not all Iranians are Persians. Adding to the confusion is that permodern writers sometimes used "Persian" as a blanket term for Iranians.^2
The 'Iranian' in your book who traveled to the Tang court was probably Soghdian, an Iranian people who historically inhabited parts of Central Asia and played a major role in the Silk Road. The former Soviet Republics of Uzbekistan and Turkmenistan were once predominately Iranian and were only Turkified after a series of invasions from the Eurasian steppe, including by the Seljuqs and Mongols.
Not to be confused with "Iranian" as in "citizens of Iran."
Patricia Crone, The Nativist Prophets of Early Islamic Iran, p. 9.