Charles Ellicott 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)

In the book. — It is doubtful what “the book” here means. The Vulgate interprets it as the Book of Esther itself, and many modern scholars concur. Still, “the book” hardly seems a natural Hebrew way for an author to refer to his work as he is writing it, and no similar case can be cited. Others think it must have been a book written at the time on the subject of the festival, which is, perhaps, possible. Canon Rawlinson identifies it with “the Book of the Chronicles of the Kings of Media and Persia,” because such is the use of the word book elsewhere in Esther.