Advanced Combat Techniques
Increases the defense and strength of all villagers 'Contains offensive and defensive fighting techniques'
one book per world
It only bites during a Blood Moon and it is one of the rarest things in the water. You can pull as many copies as you like, but the moment one is read the world stops offering them, so hoarding is pointless. Read it whenever you find it.
is it worth fishing for
Town NPCs go from decoration that occasionally throws something to a defense line that actually holds. That matters if you build your base as a fortress, if you fight invasions at home, or if you want the Nurse and the Dryad to survive a Blood Moon without babysitting. If you fight everything out in a remote arena, the effect is close to invisible.
practical notes
Blood Moon fishing is miserable without preparation, so bring a Sonar Potion and stop reeling in every catch just to check what it is. The bonuses add on top of the second volume and on top of the boosts your NPCs already picked up from bosses you have killed, and the order you read them in makes no difference.