It targets 1 face-up monster on your opponent's side of the field.
If you activate this card during the Damage Step, it can be activated from the start of the Damage Step until before damage calculation. (In that case, the attack occurring during that Damage Step continues as normal.)
Even if "Book of Moon" is activated in a Chain to the activation of "Shadow Spell", so that the targeted monster is flipped to face-downDefense Position, the effect of "Shadow Spell" is applied normally. In this case, the monster is in face-down Defense Position, and as a result only the effect that prevents it from changing its battle position is applied, but if that monster is flipped face-up by effects like "Ceasefire", the effects that reduce the ATK of that monster by 700 and prevent it from attacking are also applied, so that all of the effects of "Shadow Spell" are applied normally.
If "Book of Moon" is activated targeting a monster that is already affected by the effect of "Shadow Spell", flipping it to face-down Defense Position, the monster is no longer targeted by the effect of "Shadow Spell". (It can attack or change its battle position.) Furthermore, "Shadow Spell" remains in the Spell & Trap Zone without a target.
If you control "Noble Knight Drystan" and "Noble Knight Gawayn" and the opponent targets "Noble Knight Gawayn" with "Shadow Spell" so that the ATK of "Noble Knight Gawayn" is reduced so that it becomes lower than 1800, the effect that prevents the opponent from targeting other monsters with less than 1800 ATK you control for an attack or effects is applied, but the effect of the "Shadow Spell" that is already being applied is not affected. Afterwards, further attacks or effects cannot target "Noble Knight Gawayn".