Ibby's Magic Weekend by Heather Dyer
When straight-arrow Ibby visits her two troublemaking cousins in their chaotic country house, she learns of an old box of magic tricks they found hidden in the attic. Ibby thinks magic is nothing but sleight of hand...until her cousin Francis shrinks to the size of her thumb! The trio gets into all sorts of hijinks, from levitating to turning invisible. Then they uncover the mystery of Uncle Godfrey, a professional magician who disappeared years ago. After some sleuthing, the kids decide Godfrey's Vanishing Act must have gone wrong. Now it's up to Ibby to be the magician-- and find him!