The second book in the series, The Orphan of the Flames basically picks up shortly after book one finishes.
Brodie is struggling, but Hunter and Tusia are there to help her. In this book they meet up with Sheldon, who has his own special gift to contribute to their ongoing search. The group, which does include adults (just so you know) must work together and stay ahead of Level 5 (the organisation trying to stop them succeeding).
The adventure continues, as does the running and trying to solve the puzzles to unlock the next clue, which ultimately will help them read the code. There's a reason the code is so well hidden, but that's what they need to work out.
One of the things I really found fascinating was the secret postal line that the author claims is a real thing beneath the streets of London. It was used during the war years to deliver messages and has been closed for years now ... or has it?! Even Level 5 felt like a real thing, and possibly could be. These are a couple of reasons why I've been enjoying this series. The little intrigues behind the stories. I guess these things helped to bring the story alive.