One day Billie and Jack find a note hidden under a rock at school. It seems to be written in code! It's time the two friends put their code-breaking skills to the test.

Sally Rippin is a best-selling writer for children, with over ten million books sold in fourteen languages. Her most popular titles include the Billie B Brown and Hey Jack! series, and more recently, the School of Monsters series for beginner readers. Sally loves to write stories with heart featuring characters that resonate with children, and travels across the world speaking with parents, teachers and librarians about how to engage struggling readers.