Book of Moon

Per-phrase breakdown

• Target 1 face-up monster on the field; change that target to face-down Defense Position. (Effect)

Judge-facing facts

• Quick-Play Spell Card.
• Targets 1 face-up monster on either player's field.
• Cannot target Link Monsters or Monster Tokens.
• Cannot be activated during the Damage Step.

Card-specific notes from database

• If a monster equipped with an Equip Spell is flipped face-down, the Equip Spell loses its target and is destroyed (YGOResources).
• If a Trap Monster still treated as a Trap Card is targeted, it is Set in the Spell & Trap Zone. If no zones are available, it is sent to the GY (not treated as sent by a card effect) (Konami neuron).
• If a Trap Monster NOT treated as a Trap Card (e.g., Paleozoic) is targeted, it is changed to face-down Defense Position in the Monster Zone. It remains a Normal Monster but loses its applied conditions, such as being unaffected by monster effects or being banished when leaving the field (Konami neuron).
• If an Xyz Monster is changed to face-down Defense Position, its Xyz Materials remain attached (Konami neuron).
• If a monster Special Summoned from the Extra Deck is changed to face-down Defense Position, it is no longer treated as Special Summoned from the Extra Deck, though it remains a Special Summoned monster (Konami neuron).
• Changing a monster to face-down Defense Position is considered Setting a card; this card cannot be activated if an effect prevents Setting cards (Konami neuron).
• If a monster's effect is activated, and an effect that negates its effects (like Forbidden Chalice) is chained, followed by Book of Moon, the monster is flipped face-down before the negating effect resolves. Because it is face-down, its effects are not negated (Konami neuron).
• If a targeted effect that equips a card has its target flipped face-down before resolution, the effect is not applied (Konami neuron).
• If Book of Moon is chained to an effect that attaches Xyz Material to a target, the material cannot be attached to a face-down Xyz Monster (YGOResources).
• If a monster targeted by a Continuous Trap (like Shadow Spell) is flipped face-down, it is no longer targeted by that Trap, and the Trap remains on the field without a target (YGOResources).
• If Book of Moon is chained to the activation of a Continuous Trap (like Shadow Spell), the Trap still applies its effects to the face-down monster (e.g., it cannot change battle position, and if flipped face-up, the ATK reduction and attack prevention apply) (YGOResources).