Quick answer: To unlock the Enhanced Shrapnel Bomb skill in Nioh 3, you must find the Handbook: Enhanced Shrapnel Bomb located in the Rokuhara region. Access the area by traversing the Western Rokuhara Spirit Vein, then ascend the stairs and turn right to find a corpse on a house roof containing the item.

The Enhanced Shrapnel Bomb is a Ninja Martial Art in Nioh 3 that upgrades the standard Shrapnel Bomb, allowing players to deal tremendous damage to enemies within a wide blast radius. Unlike basic skills unlocked purely through point investment, this specific ability requires acquiring a physical Handbook item from the game world before it becomes available in the Ninja Skill Tree.
This guide details the exact location of the handbook and the steps required to reach it.
Handbook location and traversal
The Handbook: Enhanced Shrapnel Bomb is found in the Rokuhara map. Reaching the specific location requires navigating through the Western Rokuhara Spirit Vein, a traversal path that bridges different sections of the area.
Step 1: Locate and cross the Western Rokuhara Spirit Vein. This path serves as the primary gateway to the section of the map housing the item.
Step 2: Once you have successfully crossed the Spirit Vein, proceed up the set of steps immediately ahead.

Step 3: At the top of the steps, make a sharp right turn. Look for a nearby house where the roof is accessible.
Step 4: Jump onto the roof of the house. You will find a corpse here; loot it to obtain the handbook.

Unlocking the skill
Possessing the handbook does not automatically equip the skill. Once the item is in your inventory, you must manually learn the art to add it to your combat repertoire.
Open the in-game menu and navigate to the Ninja Skill Tree. The Enhanced Shrapnel Bomb node, found under Ninja Tool Arts, will now be selectable. Unlocking it usually costs Ninja Points, similar to other martial arts in the tree. Once learned, you can ready the jutsu at a Shrine to add the enhanced bombs to your shortcut items.