It permanently increases the stats of your monsters a long as this card is in the field.
It can only be equipped to Spellcaster-Type monsters.
Best monsters to equip
Book of Secret Arts is best to equip on Spellcaster-Type monsters with decent attack and defense. Cards such as A Man with Wdjat and Skilled Red Magician along with the Field Spell Yami can have stats that is hard to surpass regardless with their battle position.