"Spellbook Magician of Prophecy" and "Spellbook of Secrets".
Several "Spellbook" cards are based on mystical tomes and books from history.
Most of the cards in this archetype are Spell Cards. The exceptions are "Hidden Spellbook" and "Spellbook Magician of Prophecy". As such, numerous support cards like "Spellbook of Life" cannot use them for their effects, nor can they help you get or use them. "Prophecy" cards usually specify a "Spellbook" Spell Card in which they are also excluded. However "Spellbook of Secrets" is an exception, as it does not specify a Spell Card specifically, so it can still search them out.
Due to these cards searching each other out, they can be useful in Deck Thinning. Additionally, due to their effects benefiting Spellcaster-Type monsters, these cards can easily be used in a Spell Counter-based Deck for massive amounts of Spell Counters to be created. "Hidden Spellbook" can be used in this case to recycle those cards for future use. Also cards such as Spellbook of Judgment can be used to replenish your hand full of spellbook cards after you have used many during that turn.