Gangsta Granny, from Children's author David Walliams.

Our hero 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...