In the musical, it's Elphaba Thropp's book of spells, given to her by the Wizard's press secretary, Madame Morrible. Elphaba used it to put wings for the Winged Monkeys, make her broom-stick levitate, enchant Nessarose's shoes, save Boq's life by turning him into the Tin Man, and save Fiyero's life by turning him into the Scarecrow.