If you control a Spellcaster-Type monster: You can reveal 1 other "Spellbook" card in your hand, then target 1 "Spellbook" Normal Spell Card in your Graveyard, except "Spellbook of the Master"; this card's effect becomes that target's effect. You can only activate 1 "Spellbook of the Master" per turn.
You can use Spellbook of the Master to activate Spellbook cards in your graveyard bypassing their supposed Activation conditions.You copy the effect of Spellbook Library of the Crescent even if you have other Spellbook cards in the graveyard. You can copy the effect of Spellbook Library of the Heliosphere even if you have less than 5 Spellbook cards in the graveyard.
Here are some Spellbook spell cards that support the Prophecy Archetype and Spellcasters in general. You can use Spellbook of the Master to copy these card’s effects. Since most Spellbook cards have a “once per turn” restriction, Spellbook of the Master is a good way to get their effects out up to twice per turn.