Albert Barnes Commentary Esther 9:32

"And the commandment of Esther confirmed these matters of Purim; and it was written in the book." — Esther 9:32 (ASV)

As “the book” elsewhere in Esther always means a particular book—“the book of the chronicles of the kings of Media and Persia” (Esther 2:23; Esther 6:1; Esther 10:2)—it seems best to give it the same sense here.