Where would a book on flags of the world go Dewey decimal system?

A book on flags of the world would likely fall under the Dewey Decimal System category of 929.9.

Here's why:

* 900: This category encompasses history and geography.

* 920: This category covers biography and genealogy.

* 929: This category covers heraldry, genealogy, and related subjects.

* 929.9: This specific category focuses on flags, emblems, and insignia.

Therefore, 929.9 is the most appropriate Dewey Decimal System category for a book on flags of the world.