The spellbook of protection provides a useful spell that decreases your AC, but only temporarily. The exact reduction depends on your experience level, the amount of protection already gained from this spell, and your AC before casting the spell. The protection wears off at a rate of 1 point every 10 turns, or 1 point every 20 turns if the spell was cast with expert proficiency. The spell is unrelated to protection from priests or praying.