Which prophet was the Ethiopian official reading in his chariot when Philip met him?

Answer: Isaiah

Returning from worship in Jerusalem, the Ethiopian was sitting in his chariot reading Isaiah the prophet (KJV: Esaias).

“So he started out, and on his way he met an Ethiopian eunuch, a court official in charge of the entire treasury of Candace, queen of the Ethiopians. He had gone to Jerusalem to worship, and on his return was sitting in his chariot reading Isaiah the prophet.”— Acts 8:27-28 (BSB)