Gangsta Granny. David Walliams

Ben is bored beyond belief after he is made to stay at his grandma's house. She's the boringest grandma ever: all she wants to do is to play Scrabble, and eat cabbage soup. But there are two things Ben doesn't know about his grandma. 1) She was once an international jewel thief. 2) All her life, she has been plotting to steal the Crown Jewels, and now she needs Ben's help.