Can add any Spellbook card from your deck to your hand.
Becomes a dead card if you don’t have anymore Spellbook cards in your deck.
Spellbook of Secrets is the search card for all the other Spellbook cards available. Some might not be all that useful, but Spellbook of Life is good for bringing back your monster that’s in your grave, and Spellbook of the Master can become a copy of Spellbook of Secrets for another card search and to send some Spellbook cards to the grave for some follow-up plays that use them.
Since Spellbook cards are good at searching the other Spellbook cards, it can help with using your cards and filter through your deck very quickly, especially in combination with cards like Spell Power Grasp and Gather Your Mind. Cerberus will be able to increase its attack very quickly as you use up the cards in your deck. This makes Cerberus decks very useful to farm certain LDs quite fast.
You can rack up some spell counters pretty easily using the Spellbook cards since the help search for each other. Magical Citadel of Endymion can store some spell counters, and lets your other monsters use the counters from it, or use it to special summon Endymion, the Master Magician. Magical Exemplar is a level 4 monster that lets you special summon monsters from your hand depending on how many spell counters you have on it. Having a lot of spell counters means that you can special summon high level monsters without even needing to tribute. The skilled magicians are if you are using the decks with monsters that they can special summon. Useful to bring, especially since Red and Blue can activate their effects from the grave to give a counter to your other cards.