Answer: Qur'an
Explanation: Certainly the most important book in Islam and a major guide for Muslims is the Qur'an. According to Islamic tradition, the Prophet Muhammad had visions presented to him by an angel in a cave in the form of verses. Muhammad recorded these verses, which were later recorded in the Qur'an.
As with other religions that have other religious books and records in addition to the main holy book, so there are other important books in Islam such as Hadith, which contain sermons or sayings of Muhammad.