The Book of Spells
From the Super Mario Wiki
The Book of Spells is a book that Cybele owned but was stolen by Bowser in Mario and the Incredible Rescue. When Bowser stole it, he found out the spell to get ghosts as part of his army. Cybele tells Mario, Luigi and Toad to use six special Mushrooms to get rid of the ghosts in Bowser's army. At the end of the book, Bowser is thrown out of Bowser's Keep and Mario gets the Book of Spells. Somehow, Cybele is able to know Mario got her book back and she smiles. It is presumed it is returned to Cybele but the book ends right there and it is a mystery what Mario did to the book.