Abdulhussain Musawi in his famous essay “al-Murajiat” (number 110)  said:

They were rendered indispensable for both theoretical and practical knowledge so much so that a group of the nation’s scholars and emissaries of the Imams summarized their contents in special books to facilitate their comprehension by students and make them more accessible. The best among such compilations are the four books which are the major sources for the Imamites in referring to the roots and branches of their faith. They have been referring to them since the first century of Islam and these are: Al-Kafi Al-Tahthib Al-Istibsar and Man la Yahdaruhul Faqih. All are sequentially narrated and the authenticity of their contents is never doubted.Al-Kafi is the oldest among them the greatest the best and the most authentic.

And here scan from Arabic edition: