What is the oldest book about the interpretation of the Quran?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The oldest interpretation of the Quran, as far as we know after research, is the interpretation of the meaning of the Quran by Al-Faraa’ who died in 207 A.H.  

Allaah Knows best.