The gnome with the Knobbly knees by Rebecca Lisle

If only Clinky Monkey the dog could speak — he’d tell everyone about the garden gnome creeping round the house. But he can’t. It’s not until they catch the gnome sneaking into Laurie’s bedroom that the three brothers find out about the amazing Great Gnome Robbery masterminded by tiny Mr. Gribble, the strange man next door. Using clever disguises, the boys devise a cunning plan to foil Gribble’s plot.