Esther 10:2 (ESV)

2 And all the acts of his power and might, and the full account of the high honor of Mordecai, to which the king advanced him, are they not written in the Book of the Chronicles of the kings of Media and Persia?

The book of Esther is no fable. The early readers of the book could verify its accounts by reading the annals of the kings of Media and Persia. These annals were probably also used as a source for the writing of the book (see Esther 2:23).