What did King Xerxes have read to him on a night when he could not sleep?

Answer: The book of the chronicles

The sleepless king had the Book of Records read and learned that Mordecai had never been honored for exposing the plot.

“That night sleep escaped the king; so he ordered the Book of Records, the Chronicles, to be brought in and read to him.”— Esther 6:1 (BSB)