You can Summon “Moisture Creature” by Tributing 2 monsters, but the effect will not be activated.
You cannot Summon “Moisture Creature” by Tributing 3 monsters if there are no Spell or Trap Cards on your opponent’s side of the field.
If you activate the effect of “Moisture Creature” and your opponent chains “Book of Moon” to the effect, the effect still resolves and all Spell and Trap Cards on your opponent’s side of the field are destroyed.