My Naughty Little Sister and Bad Harry

My naughty little sister is the naughtiest little girl in the world, and there's double trouble with her best friend, Bad Harry! There's a peculiar pile of green mouldy crusts on the floor - where has it come from? Bad Harry gets covered in polish and sawdust, but he doesn't think that's naughty at all. And why on earth are those children taking pieces of cheese and bacon rind to the library?