"In the final chapter of the #BookofEsther, the book itself is described as an #iggeret, a letter. This is an unusual term for a work that ultimately would become part of #Scripture. Rather than being presented as a formal #sefer or #scroll, the #megillah is cast as a letter of correspondence.

Because #MegillatEsther is framed as an iggeret, #Halacha treats it with greater flexibility. The requirements governing how it is written and how it is read aloud are less rigid than those of other books of Scripture. These leniencies reflect its origins as a letter sent to a dispersed nation to formalize the practices of #Purim and secure them for the future, and it is therefore preserved in #Jewish law in that same form.

The status of the megillah as a letter carries deep symbolism."

https://www.jpost.com/opinion/article-887977

How letters preserved Jewish identity across empires, exile | The Jerusalem Post

How the Book of Esther uses letters to safeguard Jewish identity and language during exile and empire.

The Jerusalem Post | JPost.com
In Praise of Righteous Vengeance

On the Book of Esther, retributive violence, and the recurrence of those who seek to destroy those who won’t obey them

The Sword and the Sandwich