Upon activation, the player raises the book and gains a temporary 10 second (which translates to 300 frames) invincibility from all damage, barring the loss of heart containers.
- As with all invincibility granting items, this item allows for farming of Blood Donation Machines and Devil Beggars, free use of Sacrifice Rooms, and free entrance to or exit from the Curse Room while active.
- Changing the room while the shield is up will remove the effect.
- As with most Activated Items, Items such as Habit, 9 Volt and The Battery decrease the charge time, increasing the effectiveness of this item.
- The Virus works extremely well with this item as walking into enemies while invincible will still apply the poison and damage effects, being able to cause massive damage to high health enemies such as Isaac (boss) and ??? (boss) during the invincibility duration.
- IPECAC works very well with this item. Activating the item and melee using your tears will near instantly kill most if not all of the bosses
- Works even better with the IV Bag because of the quick recharge time, allowing for more money to be gathered without damage.
- Using this item as ??? and then picking up a devil room item will not cost you any soul hearts. You can essentially gain soul hearts by picking up The Mark or The Pact.
- The Book of Shadows, found within the Neopagan religion of Wicca, is a book containing religious texts and instructions for magical rituals.
- The symbol depiced in front of the book is called a Triquetra, a pagan symbol for trinity.
- This may be a reference to the TV series Charmed, in which the Book of Shadows had the Triquetra on the front.