The Little Secret — Kate Saunders

Jane is spending the summer with her odd new friend, Staffa, going to parties and wearing fancy dresses. Staffa's slightly scary mother, Lady Matilda, tells crazy stories, drinks a vile-smelling tea, and never strays far from a glowing painted box that the girls are under strict orders never, ever to open. But Jane can't resist; upon opening it, she is sucked into an amazing and terrifying new world so small that insects are the size of horses. Jane knows she must escape immediately or be trapped forever. But first, she has to wrangle a flying bumblebee, rescue the king, and free the people of Eck.