This book has two mysteries: The Clue of the Left-Hand Envelope and The Puzzle of the Pretty Pink Handkerchief. In the first mystery, someone sent Amber a secret-admirer letter, and Noelle and Todd want to find out who did it. In the second, Todd uses clues to find out which one of his schoolmates has been in his tree Sonlight resource: https://www.sonlight.com/products/third-grade-detectives-1-and-2