[mctt]

Not a book, but a 'fact'book; https://www.cia.gov/the-world-factbook/countries/iran/

Religions

Muslim (official) 98.5%, Christian 0.7%, Baha'i 0.3%, agnostic 0.3%, other (includes Zoroastrian, Jewish, Hindu) 0.2% (2020 est.)

Compared to the United Kingdom; https://www.cia.gov/the-world-factbook/countries/united-king...

Religions

Christian (includes Anglican, Roman Catholic, Presbyterian, Methodist) 59.5%, Muslim 4.4%, Hindu 1.3%, other 2%, unspecified 7.2%, none 25.7% (2011 est.)