These are biggest books in Islam in the order of authenticity and importance.
1. QUR’AN – revelations of Allah recited by Prophet Muhammad.
2. Sahih Bukhari – hadith collections compiled by Imam Bukhari.
3. Sahih Muslim – hadith collections compiled by Imam Muslim.
4. Sunan Abu Dawud – hadith collections compiled by Imam Abu Dawud.
5. Jami’ at-Tirmidhi – hadith collections compiled by Imam at-Tirmidhi.
6. Sunan al-Nasa’i – hadith collections compiled by Imam an-Nasai.
7a. Sunan ibn Majah – hadith collections compiled by Imam ibn Majah.
7b. Maliks Muwatta – hadith collections compiled by Imam Malik.
Qur’an is the sayings of Allah. Hadith are reports of the teachings, deeds and sayings of Prophet Muhammad. Hadiths and sira (biography) of Muhammad constitute the Sunnah. The Sunnah is the practical demonstration of the teachings of Prophet Muhammad. Sunnah is the prescribed way of life for Muslims. Allah commands Muslims to obey Muhammad in Qur’an 4:59 – O you who believe! Obey Allah and obey the Messenger (Muhammad).
Muslims are obliged to follow the authentic teachings of Muhammad as recorded in Hadith.
There are five pillars of Islam: declaration of faith, prayer, charity, fasting, and pilgrimage and without the hadith; Muslims wouldn’t know how to perform any of them. Indeed, all necessary details of the five pillars are given in the Hadith books, not in the Qur’an, therefore, sharia (Islamic laws) constitute of Qur’an and Sunnah.