Spellbook of Secrets is a Normal Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Spellbook of Secrets is Unlimited.
YGOPRODeck lists Spellbook of Secrets for these formats: Duel Links, Common Charity, TCG, OCG, Master Duel.
Spellbook of Secrets belongs to the Spellbook archetype—use that tag when searching for engine pieces and tech options.
A listed print of Spellbook of Secrets appears in Battles of Legend: Light's Revenge (BLLR-EN075) at Ultra Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Spellbook of Secrets is about $0.17 on TCGPlayer (prices change frequently).
