Assalamu Alaikum, there is a book attributed to Imam Ash-Shafi on tafseer. Did he really write a book on tafseer? JazakumAllahu kheirun.

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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

As far as we are aware of, there is no book about Tafseer that was written by Imaam Ash-Shaafi‘i . However, there is a book concerning the rulings of the Quran “Ahkaam Al-Quran”, which was not directly composed by Imaam Ash-Shaafi‘i himself. Rather, it was collected by Al-Bayhaqi from the writings of Ash-Shaafi‘i like Kitaab Al-Umm and others.

Allaah Knows best.