The Flower of Battle (2004)

From the book cover:

New Maneuvers and Proficiencies, to add further options to your campaign.
Updated Archery, Armor, Initiative, and Terrain rules.
New Firearm, Poison, and Weapon School sections.
A complete Mass Combat System that encourages Heroic Actions, allowing lone individuals to make a considerable difference in open warfare.
A section detailing Items of Power, objects which over time develop their own Spiritual Attributes to benefit, as well as possibly hinder, their bearer.
Over 100 Weapons, most seen for the first time, with complete descriptions and fully illustrated.