Gangsta Granny by David Walliams. Ages 7+.
Gangsta Granny (2011) is one of Walliams's most popular and touching novels. Ben is a boy who loves plumbing and loathes ballroom dancing (his parents' passion). He is forced to spend Friday evenings with his grandmother, whom he considers the most boring person alive — she smells of cabbage, plays Scrabble and talks about her bowel problems.
Then Ben finds a tin of jewellery hidden in her kitchen. When he confronts her, Granny reveals the truth: she is a retired international jewel thief, known as the Black Cat. She has stolen the Amber Room, the Cullinan Diamond and the Fabergé eggs. Together they plan one last heist: the Crown Jewels from the Tower of London.
The heist is a spectacular, funny failure. But something more important happens: Ben and his grandmother discover each other. The ending, when it comes, is genuinely moving — Granny has been keeping a secret far more important than her criminal past, and the final chapter is one of the most affecting in all of Walliams's work.