The Four Books is a Twelver Shiʿa term referring to their four best known hadith collections.

The books are:

Name	Collector	Size
Kitab al-Kafi Kitab al-Kafi The Kitāb al-Kāfī is a Twelver Shī‘ah hadīth collection compiled by Muhammad Ya‘qūb Kulaynī. It is divided into three sections: Usūl al-Kāfī, which is concerned with the principle of religion, Furū al-Kāfī, which is concerned with the details of religious law, and Rawdat al-Kāfī, which is...  (divided into Usul al-Kafi, Furu al-Kafi and Rawdat al-Kafi)	Muhammad ibn Ya'qub al-Kulayni al-Razi (329 AH Hijri year The Hijri year is year numbering system used in the Islamic calendar. It commemorates the Hijra , or emigration of Muhammad and his followers to the city of Medina in 622 CE. In Arabic, AH is symbolized by the letter هـ... )	15,176 hadith
Man la yahduruhu al-Faqih Man la yahduruhu al-Faqih Man lā yahduruhu al-Faqīh is a hadith collection, by the famous Twelver Shi'a hadith scolar Abu Ja'far Muhammad ibn 'Ali ibn Babawaih al-Qummi, commonly known as Ibn Babawaih or Al-Shaykh al-Saduq. This work is included among The Four Books of Twelver Shia Islam. Like all other Twelver Shi'a books...	Muhammad ibn Babuya Al-Shaykh al-Saduq Al-Shaykh al-Saduq is the title given to Abu Ja'far Muhammad ibn 'Ali ibn Babawaih al-Qummi. He was the leading traditionist of his time and one of the most outstanding traditionists of Shi'ite Islam. He earned the title of al-Shaykh al-Saduq on account of his great learning and his reputation for...	9,044
Tahdhib al-Ahkam Tahdhib al-Ahkam Tahdhib al-Ahkam is a hadith collection, by the famous Twelver Shi'a hadith scholar Abu Jafar Muhammad Ibn Hassan Tusi, commonly known as Shaykh Tusi...	Shaykh Muhammad Tusi Shaykh Tusi Shaykh Tusi , full name: Abu Jafar Muhammad Ibn Hassan Tusi , known as Shaykh al-Taʾifah was a prominent Persian scholar of the Shi'a Twelver Islamic belief.-Birth:...	13,590
Al-Istibsar Al-Istibsar Al-Istibsar is the fourth important book of Shi'a Islamic Hadith. It was written by Abu Ja'far al-Tusi....	Shaykh Muhammad Tusi Shaykh Tusi Shaykh Tusi , full name: Abu Jafar Muhammad Ibn Hassan Tusi , known as Shaykh al-Taʾifah was a prominent Persian scholar of the Shi'a Twelver Islamic belief.-Birth:...	5,511

Shi'a

Shi'a Islam

Shia Islam is the second largest denomination of Islam. The followers of Shia Islam are called Shi'ites or Shias. "Shia" is the short form of the historic phrase Shīʻatu ʻAlī , meaning "followers of Ali", "faction of Ali", or "party of Ali".Like other schools of thought in Islam, Shia Islam is...

 Muslims use different books of ahadith than Ahl al-Sunnah

Sunni Islam

Sunni Islam is the largest branch of Islam. Sunni Muslims are referred to in Arabic as ʾAhl ūs-Sunnah wa āl-Ǧamāʿah or ʾAhl ūs-Sunnah for short; in English, they are known as Sunni Muslims, Sunnis or Sunnites....

's Six major Hadith collections

Six major Hadith collections

The six major Hadith collections are collections of hadith by Islamic scholars who, approximately 200 years after Muhammad's death and by their own initiative, collected "hadith" attributed to Muhammad...

. The Shi'a consider many Sunni transmitters of hadith to be unreliable because many of them accepted the Caliphate

Caliphate

The term caliphate, "dominion of a caliph " , refers to the first system of government established in Islam and represented the political unity of the Muslim Ummah...

 of Abu Bakr

Abu Bakr

Abu Bakr was a senior companion and the father-in-law of the Islamic prophet Muhammad. He ruled over the Rashidun Caliphate from 632-634 CE when he became the first Muslim Caliph following Muhammad's death...

, Umar

Umar

`Umar ibn al-Khattāb c. 2 November , was a leading companion and adviser to the Islamic prophet Muhammad who later became the second Muslim Caliph after Muhammad's death....

 and Uthman

Uthman

Uthman ibn Affan was one of the companions of Islamic prophet, Muhammad. He played a major role in early Islamic history as the third Sunni Rashidun or Rightly Guided Caliph....

 in preference to Ali

Ali

' |Ramaḍān]], 40 AH; approximately October 23, 598 or 600 or March 17, 599 – January 27, 661).His father's name was Abu Talib. Ali was also the cousin and son-in-law of the Islamic prophet Muhammad, and ruled over the Islamic Caliphate from 656 to 661, and was the first male convert to Islam...

. Shia trust traditions transmitted through the Imams

Imamah (Shi'a doctrine)

Imāmah is the Shia doctrine of religious, spiritual and political leadership of the Ummah. The Shīa believe that the A'immah are the true Caliphs or rightful successors of Muḥammad, and further that Imams are possessed of divine knowledge and authority as well as being part of the Ahl al-Bayt,...

, Muhammad's descendants through Fatima Zahra.
The Four Books have been praised by many notable Shi'a scholars. This is what some have said:

• Shaikh al-Hur al-Aamili
  Shaikh al-Hur al-Aamili
  Muhammad bin al-Ḥassan b. Ali b. al-Ḥusayn al-ʿĀmili al-Mashghari , commonly known as Al-Ḥurr Al-ʿĀmili , was a muhaddith and a prominent Twelver Shi’a scholar of the Akhbari school of thought...
  
   said, "The authors of the 4 Books of shia have testified that the Hadiths of their books are accurate (saheeh) , firm and well conducted from the roots that all shia agreed on , and if you consider those scholars (the authors of the four books) are reliable then you must accept their sayings and their narrations . " [Al-wasa’el, vol. 20, p. 104]

• Abd al-Husayn Sharaf al-Din al-Musawi
  Abd al-Husayn Sharaf al-Din al-Musawi
  Abd al-Husayn Sharaf al-Din al-Musawi was a Shi'a twelver Islamic scholar.-Biography:Born 1872 CE in Kadhimiya in Iraq he left Lebanon and went back to Iraq at age 17 in order to study, were he stayed for fifteen years until he became a mujtahid at the age of 32.He then returned to his home in...
  
   said: "Al-Kafi, Al-Istibsaar, Al-Tahzeeb and Mun La Yahduruhu Al-faqeeh are Mutawatirah and agreed on the accuracy of its contents (the Hadiths), and Al-Kafi is the oldest, greatest, best and the most accurate one of them.“ [The book of Al-Muraja'aat (A Shi'i-Sunni dialogue
  A Shi'i-Sunni dialogue
  A Shi'i-Sunni dialogue , also translated as The Right Path is a book about Islam written by Sayyid Abd al-Husayn Sharaf al-Din al-Musawi....
  
  ), Muraj'ah No. 110]

• Mohammad Sadeq al-Sadr said: “Although the Shia are on the unanimity of that the four books are accepted and all the narrations in them are accurate ( Saheeh ), But they did not call them by the name (Sihaah ) like Ahl-Sunnah did." [The Book of shia "Kitab al-shia" p. 127]

• Al-Tabrosi (aka, Tabarsi) said:"Al-Kafi among the 4 shia books is like the sun among the stars, and who looked fairly would not need to notice the position of the men in the chain of hadiths in this Book, and if you looked fairly you would feel satisfied and sure that the hadiths are firm and accurate."[Mustadrak al-Wasail, vol. 3, p. 532]