The Unbreakable Code

Mr. Quisling is definitely up to something mysterious and Emily and James are on high alert. First there's a coded note he drops at the book event. Then Emily and James uncover a trail of encrypted messages in books by Mark Twain hidden through Book Scavenger. What's most suspicious is that each hidden book triggers and arson.
As the sleuthing friends dig deeper, they discover Mr. Quisling has been hunting a legendary historical puzzle: the unbreakable code. The new mystery is irresistible, but Emily and James can't ignore the signs that Mr. Quisling might be the arsonist. The clock is ticking, the fires multiplying, while Emily and James race to crack the code of a lifetime.