Nox/Equipment

Quivers
Quivers are ammo for bows (technically they hold the ammo, but arrows are not an item within the game) and thus they are, of course, required if one wishes to shoot an arrow.

Poison
Not very useful, since you'll want your target to die quickly, but if you don't want to waste mana on casting poison, equip these and fire off a clean shot before running away; or just use them to take down an enemy via impact damage. If you find the poison buff useful, then of course use them sparingly.

Fire
Fire quivers become frequent pick-ups after about Chapter 6. Each shot will explode on impact, dealing fire damage in a small area of effect. Thus, they are perfect for Zombies.

Numbing/Stun
Rare quivers, you'll want to only use these in difficult situations versus single, powerful monsters such as bosses.