We’ve seen the sacred text of Islam spelled several different ways. Which is correct?
Before we get to that, remember that that sacred works are proper nouns and are capitalized.
…Do you believe the Bible to be inerrant?
…The Koran should be kept on the highest bookshelf.
…What does the Book of Mormon say about…
If the work is referenced as an adjective, do not capitalize.
…in reference to those biblical texts…
According to MW (February 2019), the sacred work of Islam may be spelled Koran, Quran, or Qur’an.
LMEG does not reference it.
BGGP and GRM have Qur’an.
CMOS has Koran and Qur’an
GPO has Koran.
AP says use Quran unless Koran is preferred by a specific organization or in a specific title or name.